Prevent default branch from being changed if setting is present

Why are we doing this work

• if the default branch is included in an enabled policy with block_unprotecting_branches: true, the branch should not be able to be removed as the default branch

Relevant links

Non-functional requirements

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

Implementation plan

• backend add validation for if a default branch can/cannot be changed
• frontend disable the save button at `/-/settings/repository#default-branch

Verification steps

1. Upload a GitLab Ultimate license
2. Enable the feature flags

echo "Feature.enable(:scan_result_policies_block_unprotecting_branches)" | rails c

3. Go to Project => Settings => Repository => Branch defaults => Make main the default branch
4. Go to Secure -> Policies -> New policy -> Scan Result Policy
5. Go to yaml mode
6. Use this policy:

type: scan_result_policy
name: Require approval for vulnerabilities
description: ''
enabled: true
rules:
  - type: scan_finding
    scanners: []
    vulnerabilities_allowed: 0
    severity_levels: []
    vulnerability_states: []
    branch_type: protected
actions:
  - type: require_approval
    approvals_required: 1
    role_approvers:
      - maintainer
      - developer
approval_settings:
  block_unprotecting_branches: true

7. Save the policy
8. Go the Project => Settings => Repository => Branch defaults
9. Verify the default branch cannot be changed and a popover shows up over the button