Confirm database connection before running post migration

What does this MR do and why?

Related to this note !111200 (comment 1272288120)

We need to add this new line

return unless Gitlab::Database.gitlab_schemas_for_connection(connection).include?(:gitlab_main)

to db/post_migrate/20221108045019_truncate_timeline_event_tags_table.rb to fix the pipeline of JiHu.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Make sure you are running in decomposed database setup.
2. Lock the legacy tables rake gitlab:db:lock_writes.
3. Run the CI migration down and up on confirm-database-connection-before-running-post-migration branch. it should succeed.

➜  gitlab git:(confirm-database-connection-before-running-post-migration) be rake gitlab:db:lock_writes
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
➜  gitlab git:(confirm-database-connection-before-running-post-migration) be rails db:migrate:down:ci VERSION=20221108045019
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
ci: == 20221108045019 TruncateTimelineEventTagsTable: reverting ===================
ci: == 20221108045019 TruncateTimelineEventTagsTable: reverted (0.0004s) ==========

➜  gitlab git:(confirm-database-connection-before-running-post-migration) ✗ be rails db:migrate:up:ci VERSION=20221108045019
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
ci: == 20221108045019 TruncateTimelineEventTagsTable: migrating ===================
ci: == 20221108045019 TruncateTimelineEventTagsTable: migrated (0.0026s) ==========

➜  gitlab git:(confirm-database-connection-before-running-post-migration) ✗ be rails db:migrate:down:main VERSION=20221108045019
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
main: == 20221108045019 TruncateTimelineEventTagsTable: reverting ===================
main: == 20221108045019 TruncateTimelineEventTagsTable: reverted (0.0033s) ==========

➜  gitlab git:(confirm-database-connection-before-running-post-migration) ✗ be rails db:migrate:up:main VERSION=20221108045019
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
main: == 20221108045019 TruncateTimelineEventTagsTable: migrating ===================
main: -- execute("TRUNCATE TABLE incident_management_timeline_event_tags, incident_management_timeline_event_tag_links")
main:    -> 0.0223s
main: == 20221108045019 TruncateTimelineEventTagsTable: migrated (0.0232s) ==========

4. Run the CI migration down and up on master branch, it will fail.

➜  gitlab git:(master) be rails db:migrate:down:ci VERSION=20221108045019
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
ci: == 20221108045019 TruncateTimelineEventTagsTable: reverting ===================
ci: == 20221108045019 TruncateTimelineEventTagsTable: reverted (0.0006s) ==========

➜  gitlab git:(master) ✗ be rails db:migrate:up:ci VERSION=20221108045019
WARNING: This version of GitLab depends on gitlab-shell 14.17.0, but you're running 14.16.0. Please update gitlab-shell.
ci: == 20221108045019 TruncateTimelineEventTagsTable: migrating ===================
ci: -- execute("TRUNCATE TABLE incident_management_timeline_event_tags, incident_management_timeline_event_tag_links")
rails aborted!
StandardError: An error has occurred, all later migrations canceled:

PG::SREModifyingSqlDataNotPermitted: ERROR:  Table: "incident_management_timeline_event_tags" is write protected within this Gitlab database.
HINT:  Make sure you are using the right database connection
CONTEXT:  PL/pgSQL function gitlab_schema_prevent_write() line 4 at RAISE
/Users/zhzhang/working/gitlab-development-kit/gitlab/db/post_migrate/20221108045019_truncate_timeline_event_tags_table.rb:7:in `up'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb:33:in `block in exec_migration'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/query_analyzer.rb:37:in `within'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb:30:in `exec_migration'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migrations/lock_retry_mixin.rb:36:in `ddl_transaction'

Caused by:
ActiveRecord::StatementInvalid: PG::SREModifyingSqlDataNotPermitted: ERROR:  Table: "incident_management_timeline_event_tags" is write protected within this Gitlab database.
HINT:  Make sure you are using the right database connection
CONTEXT:  PL/pgSQL function gitlab_schema_prevent_write() line 4 at RAISE
/Users/zhzhang/working/gitlab-development-kit/gitlab/db/post_migrate/20221108045019_truncate_timeline_event_tags_table.rb:7:in `up'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb:33:in `block in exec_migration'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/query_analyzer.rb:37:in `within'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb:30:in `exec_migration'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migrations/lock_retry_mixin.rb:36:in `ddl_transaction'

Caused by:
PG::SREModifyingSqlDataNotPermitted: ERROR:  Table: "incident_management_timeline_event_tags" is write protected within this Gitlab database.
HINT:  Make sure you are using the right database connection
CONTEXT:  PL/pgSQL function gitlab_schema_prevent_write() line 4 at RAISE
/Users/zhzhang/working/gitlab-development-kit/gitlab/db/post_migrate/20221108045019_truncate_timeline_event_tags_table.rb:7:in `up'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb:33:in `block in exec_migration'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/query_analyzer.rb:37:in `within'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb:30:in `exec_migration'
/Users/zhzhang/working/gitlab-development-kit/gitlab/lib/gitlab/database/migrations/lock_retry_mixin.rb:36:in `ddl_transaction'
Tasks: TOP => db:migrate:up:ci
(See full trace by running task with --trace)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.