Fix approval policy to include group level protected branch

What does this MR do and why?

This MR fixes a bug with MR approval rules when allow_protected_branches_for_group & group_protected_branches feature flag are enabled and when a project does not have a branch that is specified in group level's protected branches.

MR acceptance checklist

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How to set up and validate locally

1. Create a group and enabled allow_protected_branches_for_group & group_protected_branches FFs to the group
2. Create a group level protected branch (feature) from Group -> Settings -> Repository -> Protected Branches
3. Add the following policy below to that group (Secure -> Policies -> New Policy -> Merge request approval policy)

   ---
   scan_result_policy:
   - name: Policy test
     description: ''
     enabled: true
     actions:
     - type: require_approval
       approvals_required: 1
       role_approvers:
       - maintainer
       - owner
     rules:
     - type: any_merge_request
       commits: any
       branches:
       - feature
     approval_settings:
       block_branch_modification: true
       prevent_pushing_and_force_pushing: true
       prevent_approval_by_author: true
       prevent_approval_by_commit_author: true
       remove_approvals_with_new_commit: true
       require_password_to_approve: false
     policy_scope:
       compliance_frameworks: []

4. Create a project within the group
5. Create 2 more non-protected branches. (Let's call them new-branch-1 and new-branch-2)
6. Make some changes on new-branch-2. For example, adding a new line in a README file.
7. Create an a MR with new-branch-2 as the source branch, and new-branch-1 as the target branch.
8. Observe that approval is not required for the MR

Addresses #456055