Remove race condition while deleting groups

What does this MR do?

The intended flow during a group deletion is:

  Soft-delete group (sync) -> Delete group projects (async) -> Hard-delete group (async)

The soft-delete was run in a transaction, which was committed only after the async job (for hard-deletion) was kicked off. There was a race condition here - the soft-delete transaction could complete after the hard delete completed, leaving a soft-deleted record in the database.

This MR removes this race condition. There is no need to run the soft-delete in a transaction. The soft-delete completes before the async job is kicked off.

This MR also adds a migration to delete all existing (soft-deleted) groups left in an inconsistent state due to this bug.

Does this MR meet the acceptance criteria?

• [#23223 (closed)/!7528 (merged)] Groups not getting fully deleted / 500
  • Implementation
    • Fix the race condition
    • Write a migration to remove all groups currently in limbo (marked for deletion but not deleted)
  • Tests
    • Added
      • Race condition fix
    • Test migration on staging
    • Passing
  • Meta
    • CHANGELOG entry created
    • Documentation created/updated
    • API support added
    • Branch has no merge conflicts with master
    • Squashed related commits together
  • Review
    • Endboss
  • Wait for merge

What are the relevant issue numbers?

• Closes #23223 (closed)
• EE merge request: gitlab-org/gitlab-ee!886