Allow multiple any_merge_request rules to be created

What does this MR do and why?

Allow multiple merge request approval rules of report_type: any_merge_request to be created in the same way as scan_finding and license_scanning rules.

If there are multiple policies with any_merge_request type, these must correspond to multiple approval rules. Currently, only one any_merge_request approval rule is created for a merge request.

This feature behind a feature flag scan_result_any_merge_request which is disabled by default, hence no changelog entry.

While working on a test for this change, I noticed that existing test was broken in many places and not executed at all, so I fixed that too.

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:scan_result_any_merge_request)

2. Create two policies with any_merge_request type. Sample YAML:

   type: scan_result_policy
   name: Any commits
   description: ''
   enabled: true
   rules:
     - type: any_merge_request
       branch_type: protected
       commits: any
   actions:
     - type: require_approval
       approvals_required: 1
       user_approvers_ids:
         - 1 # replace with a valid user ID

3. Create MR in the project and verify that both rules are applied

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 #418752 (closed)