Convert group input box into dropdown selection

What does this MR do and why?

Convert group input box into dropdown selection

• replace input box with dropdwon
• retrieve top-level groups only
• disable dropdowns when disabled
• add tests

References

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MR acceptance checklist

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Screenshots or screen recordings

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Scenario	Before	After
Disabled
Enabled w/ exceptions
errors	-

How to set up and validate locally

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

1. Upload a GitLab Ultimate license
2. Enable the feature flag echo "Feature.enable(:scan_result_policy_block_group_branch_modification)" | rails c
3. Navigate to a group => Secure => Policies => New policy => Merge request approval policy
4. Verify the Block group branch modification setting shows and can be toggled and modified
5. Save the policy
6. Navigate to a group => Secure => Policies => Select the new policy => Edit policy
7. Verify the Block group branch modification setting shows and can be toggled and modified

Related to #500842 (closed)