Move `push_rule.rb` into the `Repositories` namespace
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=321507) </details> <!--IssueSummary end--> ## Context The following discussion from !54113 should be addressed: - [ ] @pslaughter started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/54113#note_508683113): > **Why did we add this to `rubocop_manual_todo.yml`?** > > Without this, rubocop fails with: > > ![Screen_Shot_2021-02-13_at_2.35.18_PM](/uploads/dac830c942469be99a8f3a95b7be455d/Screen_Shot_2021-02-13_at_2.35.18_PM.png) > > The [`declarative_policy.rb` module](https://gitlab.com/gitlab-org/gitlab/blob/b287e3544ffdaa9c68f5b8569e4e53c8c6969d25/lib/declarative_policy.rb#L74) tries to resolve this policy class dynamically. For it to do so, the policy class needs to be in the same namespace as the corresponding class. I tried moving it into it's own namespace and use `declarative_policy_class` on `push_rule.rb`, but this also didn't work for me :grimacing:
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