Prevent delete protected branches via security policies

What does this MR do and why?

This prevents the deletion of a protected branch on the project level when a security policy enables the block_unprotecting_branches setting as described in #420728 (closed)

Note: This MR only addresses the backend changes. I suggested hiding the Delete protected branch button here to avoid confusion.

How to set up and validate locally

1. Enable the feature flag Feature.enable(:scan_result_policies_block_unprotecting_branches)
2. Create a project
3. Create a scan result policy via Secure -> Policies -> New policy
4. Switch to .yaml mode and paste the following yaml:

   type: scan_result_policy
   name: Container Scanning
   description: ''
   enabled: true
   rules:
     - type: scan_finding
       scanners:
         - container_scanning
       vulnerabilities_allowed: 0
       severity_levels: []
       vulnerability_states: []
       branch_type: protected
   actions:
     - type: require_approval
       approvals_required: 1
       user_approvers_ids:
         - 1
   approval_settings:
     block_unprotecting_branches: true

5. Select Configure with a merge request and merge the MR
6. Create a new branch on the project
7. Make the branch protected via Settings -> Repository -> Protected branches
8. Go to Code -> Branches and try to delete the protected branch
9. This should fail with an error message saying: "Deleting protected branches is blocked by security policies"

Screenshots

Screenshot

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 #420728 (closed)