Revert "Merge branch '299290-fj-remove-unamespaced-git-garbage-collect-worker' into 'master'"

What does this MR do?

Reverts a perfectly good MR !61892 (merged) that I merged inadvertently too early, which was caught in the release for %13.12 instead. Workers can only be removed in major versions.

Release task: gitlab-org/release/tasks#2486 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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