Disable change default button

What does this MR do and why?

Disable change default option

• for branches that are protected by a security policy
• add popover explaining why it was disabled

Screenshots or screen recordings

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

Before	After

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 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: default
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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #430304 (closed)