Add block group branch modification setting

What does this MR do and why?

Add block group branch modification setting

• show in list for groups
• hide at project level

Changelog: added

EE: true

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Scenario	Screenshot
Project-level
Group-level w/ ff off	No change
Group-level w/ ff on
Group-level w/ ff on - new policy
Group-level w/ ff on - existing policy w/out setting
Group-level w/ ff on - existing policy w/ setting

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 project => Secure => Policies => New policy => Merge request approval policy
4. Verify the Block group branch modification setting does not show
5. Navigate to a group => Secure => Policies => New policy => Merge request approval policy
6. Verify the Block group branch modification setting shows and can be toggled and modified
7. Create the policy
8. Navigate to the group => Secure => Policies => Select the new policy => Edit policy
9. Verify the policy editor displays the setting as enabled with the exceptions
10. Navigate to the group => Secure => Policies => Select a different policy => Edit policy
11. Verify the policy editor displays the setting as disable

Related to #435725