Include subgroups for protected env group dropdown

What does this MR do and why?

This MR implements #425840 (closed) by showing subgroups of a given group in selected groups dropdown.

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create group main
2. Assign the group to Ultimate plan (view the group in admin area, and select an Ultimate plan in Permissions and group features section.
   Click to expand

   
3. Create sub-group users
4. Create sub-subgroups developers and deployers of users group
5. Navigate to group level CI/CD settings for `main group
6. Create new protected environment
7. Check that all three groups are selectable in the protected environment (users, developers and deployers)