Avoid creating approval rules from policies for unprotected branches

What does this MR do and why?

This MR solves a problem that existed when the user tried to create MR for non-protected branches, with group-level merge request approval policies defined.

References

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• Security Policy blocks merge even if the condit... (#503227 - closed)

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Before	After

How to set up and validate locally

1. Create new Group
2. Create new Policy for the group:

name: Policy
description: Merge request policy to apply to all projects
enabled: true
actions:
- type: require_approval
  approvals_required: 1
  user_approvers_ids:
  - 55
  - 65
  - 62
  - 1
- type: send_bot_message
  enabled: true
rules:
- type: any_merge_request
  commits: any
  branches:
  - main
- type: any_merge_request
  commits: any
  branches:
  - master
- type: any_merge_request
  commits: any
  branches:
  - prod
- type: any_merge_request
  commits: any
  branches:
  - production
approval_settings:
  block_branch_modification: true
  block_group_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:
  projects:
    excluding: []
fallback_behavior:
  fail: closed

3. Create new project in that group
4. Create new MR where both source and target branches are not protected
5. Observe that approval is not required

Related to #503227 (closed)