Drop all background migrations in the next major release
This issue is a thought that sprouted from a discussion in !43307 (merged).
Proposal
From https://docs.gitlab.com/ee/update/#upgrading-to-a-new-major-version:
It’s also important to ensure that any background migrations have been fully completed before upgrading to a new major version. To see the current size of the background_migration queue, Check for background migrations before upgrading.
So technically it means we can delete all the code in lib/gitlab/background_migration/
in %14.0. This will clean up the codebase and gives us less code to maintain.