Don't cleanup all doc folders in gems, just the top level

What does this MR do?

Fixes an issue where we might delete valid ruby code under any doc directory.

The cleanup script already has a step where we remove doc from the top level of a gem, but we also had this final step that cleaned up all docs. This was helping us in removing some extras from the extensions like seen here: https://gitlab.com/gitlab-org/build/CNG/-/jobs/2444767262#L2123

As far as I can see that one extension was the only additional doc folder being cleaned up.

But in https://gitlab.com/gitlab-org/build/CNG-mirror/-/jobs/2444622263#L1379 we can see it removing some necessary gem code.

Checklist

See Definition of done.

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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated