Rename setting to `block_branch_modification`

Why are we doing this work

• &9705 (comment 1650392640)

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• frontend rename block_unprotecting_branches to block_branch_modification
• backend rename block_unprotecting_branches to block_branch_modification

Verification steps

1. Upload a GitLab Ultimate license
2. Navigate to a project => Settings => Repository => Protected Branches
3. Protect several branches
4. Navigate to Secure => Policies => New policy => Scan Result Policy
5. Create a policy with some of the protected branches selected (or some excepted) and the following settings

approval_settings:
  block_branch_modification: true

6. Save the policy
7. Navigate to the project => Settings => Repository => Protected Branches
8. Verify the affected branches cannot be unprotected
9. Navigate to Settings => Repository => Branch defaults
10. Verify if the default branch is included in the policy, that the default branch cannot be changed
11. Navigate to Code => Branches
12. Verify the affected branches cannot be deleted