Define BoundedContext for RemoteMirrors
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See documentation: https://docs.gitlab.com/ee/development/software_design.html#how-to-define-bounded-contexts
New set of [`RemoteMirrors` services](https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/services/remote_mirrors) was implemented before `BoundedContext` restriction was defined.
We need to fix Rubocop warnings for them.
```
Gitlab/BoundedContexts: Module RemoteMirrors is not a valid bounded context. See https://docs.gitlab.com/ee/development/software_design#bounded-contexts.
```
issue