Draft: Finalize PK conversion for push_event_payloads only for GitLab.com

What does this MR do?

We found that the Database::BatchedBackgroundMigrationWorker is running only for GitLab.com. This mean that even if we though we shipped PK conversion batched migrations for self-managed, they are not being processed, and self-managed instances are not ready to finalize PK conversions in 14.1 - ensure_batched_background_migration_is_finished will be failing for them.

This MR is fixing this problem by:

1. Updating the existing migration db/post_migrate/20210622041846_finalize_push_event_payloads_bigint_conversion.rb to be executed only on GitLab.com (+ dev and test) environments. This will diverge the database schema between existing self-managed and GitLab.com. New self-managed installations will have the same schema as GitLab.com.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #288005 (closed)