Add security policy schema to block unprotecting branches

What does this MR do and why?

This adds security policy schema for the "block unprotecting branches" setting.

How to set up and validate locally

1. Create a project
2. Create a scan result security policy with the following content:

   type: scan_result_policy
   name: Any unsigned MR
   description: ''
   enabled: true
   rules:
     - type: any_merge_request
       branch_type: protected
       commits: unsigned
   actions:
     - type: require_approval
       approvals_required: 1
       user_approvers_ids:
         - 4 # Change to any existing user ID with access to the project
   approval_settings:
     block_unprotecting_branches: true

3. Copy the ID of your project
4. Start a rails console
5. Fetch the Security::ScanResultPolicyRead:

   Security::ScanResultPolicyRead.where(project_id: 172).first.project_approval_settings

6. It should have the following value:

   {"block_unprotecting_branches"=>true}

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 #423101 (closed)