Add post migration step for removing null pages_domain

What does this MR do and why?

Add post migration step for removing null pages_domain

Local migration logs

➜  gitlab git:(master) ✗ bin/rails db:migrate RAILS_ENV=development               
main: == [advisory_lock_connection] object_id: 128800, pg_backend_pid: 34997
main: == 20240916084832 DeleteInvalidPagesDomainsRecords: migrating =================
main: == 20240916084832 DeleteInvalidPagesDomainsRecords: migrated (0.0400s) ========

main: == [advisory_lock_connection] object_id: 128800, pg_backend_pid: 34997
ci: == [advisory_lock_connection] object_id: 129040, pg_backend_pid: 34999
ci: == 20240916084832 DeleteInvalidPagesDomainsRecords: migrating =================
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_internal, :gitlab_shared].
ci: == 20240916084832 DeleteInvalidPagesDomainsRecords: migrated (0.0073s) ========

ci: == [advisory_lock_connection] object_id: 129040, pg_backend_pid: 34999

➜  gitlab git:(master) ✗ bin/rails db:migrate RAILS_ENV=development                         
main: == [advisory_lock_connection] object_id: 128800, pg_backend_pid: 36099
main: == 20240916091255 AddNotNullConstraintToPagesDomainProjectId: migrating =======
main: -- current_schema(nil)
main:    -> 0.0012s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE pages_domains\nADD CONSTRAINT check_790fbb64fa\nCHECK ( project_id IS NOT NULL )\nNOT VALID;\n")
main:    -> 0.0016s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- execute("ALTER TABLE pages_domains VALIDATE CONSTRAINT check_790fbb64fa;")
main:    -> 0.0006s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20240916091255 AddNotNullConstraintToPagesDomainProjectId: migrated (0.0390s) 

main: == [advisory_lock_connection] object_id: 128800, pg_backend_pid: 36099
ci: == [advisory_lock_connection] object_id: 129060, pg_backend_pid: 36101
ci: == 20240916091255 AddNotNullConstraintToPagesDomainProjectId: migrating =======
ci: -- current_schema(nil)
ci:    -> 0.0002s
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- execute("ALTER TABLE pages_domains\nADD CONSTRAINT check_790fbb64fa\nCHECK ( project_id IS NOT NULL )\nNOT VALID;\n")
ci:    -> 0.0018s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- execute("ALTER TABLE pages_domains VALIDATE CONSTRAINT check_790fbb64fa;")
ci:    -> 0.0008s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20240916091255 AddNotNullConstraintToPagesDomainProjectId: migrated (0.0202s) 

ci: == [advisory_lock_connection] object_id: 129060, pg_backend_pid: 36101

Related: #442178 (closed)

Changelog: changed

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