Add missing restrict_gitlab_migration to migration

For instances who have pending background migrations, we need to declare this migration as DML for main. Because this background migration will run DML queries for projects, and merge_request_diff_commits table.

Added specs which show the error:

     Gitlab::Database::QueryAnalyzers::RestrictAllowedSchemas::DMLNotAllowedError:
       Select/DML queries (SELECT/UPDATE/DELETE) are disallowed in the DDL (structure) mode. Modifying of '["projects"]' ([:gitlab_main]) with 'SELECT "projects"."id", "projects"."name", "projects"."path", "projects"."description", "projects"."created_at", "projects"."updated_at", "projects"."creator_id", "projects"."namespace_id", "projects"."last_activity_at", "projects"."import_url", "projects"."visibility_level", "projects"."archived", "projects"."avatar", "projects"."merge_requests_template", "projects"."star_count", "projects"."merge_requests_rebase_enabled", "projects"."import_type", "projects"."import_source", "projects"."approvals_before_merge", "projects"."reset_approvals_on_push", "projects"."merge_requests_ff_only_enabled", "projects"."issues_template", "projects"."mirror", "projects"."mirror_user_id", "projects"."shared_runners_enabled", "projects"."runners_token", "projects"."build_allow_git_fetch", "projects"."build_timeout", "projects"."mirror_trigger_builds", "projects"."pending_delete", "projects"."public_builds", "projects"."last_repository_check_failed", "projects"."last_repository_check_at", "projects"."only_allow_merge_if_pipeline_succeeds", "projects"."has_external_issue_tracker", "projects"."repository_storage", "projects"."repository_read_only", "projects"."request_access_enabled", "projects"."has_external_wiki", "projects"."ci_config_path", "projects"."lfs_enabled", "projects"."description_html", "projects"."only_allow_merge_if_all_discussions_are_resolved", "projects"."repository_size_limit", "projects"."printing_merge_request_link_enabled", "projects"."auto_cancel_pending_pipelines", "projects"."service_desk_enabled", "projects"."cached_markdown_version", "projects"."delete_error", "projects"."last_repository_updated_at", "projects"."disable_overriding_approvers_per_merge_request", "projects"."storage_version", "projects"."resolve_outdated_diff_discussions", "projects"."remote_mirror_available_overridden", "projects"."only_mirror_protected_branches", "projects"."pull_mirror_available_overridden", "projects"."jobs_cache_index", "projects"."external_authorization_classification_label", "projects"."mirror_overwrites_diverged_branches", "projects"."pages_https_only", "projects"."external_webhook_token", "projects"."packages_enabled", "projects"."merge_requests_author_approval", "projects"."pool_repository_id", "projects"."runners_token_encrypted", "projects"."bfg_object_map", "projects"."detected_repository_languages", "projects"."merge_requests_disable_committers_approval", "projects"."require_password_to_approve", "projects"."emails_disabled", "projects"."max_pages_size", "projects"."max_artifacts_size", "projects"."remove_source_branch_after_merge", "projects"."marked_for_deletion_at", "projects"."marked_for_deletion_by_user_id", "projects"."autoclose_referenced_issues", "projects"."suggestion_commit_message", "projects"."project_namespace_id", "projects"."hidden" FROM "projects" WHERE "projects"."id" = $1 LIMIT $2 /*application:test,correlation_id:b96998229f17d6a0bd6e1bbe3b32972a,db_config_name:main*/'. For more information visit: https://docs.gitlab.com/ee/development/database/migrations_for_multiple_databases.html

Note this migration run fine on production because on GitLab.com there were no pending background_migration_jobs to process.