Move `push_rule.rb` into the `Repositories` namespace
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## 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:
>
> 
>
> 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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