Create v2 approval rules in creations controller behind ff

What does this MR do and why?

Add v2 approval rules in the mr creations controller as part of rearchitecting approval rules. When the v2_approval_rules flag is enabled rules are dual written in both the existing and new architecture.

How to set up and validate locally

1. Enable the v2_approval_rules feature flag.
2. Create a new merge request with a MR level approval rule.
3. Check that a new row has been added to the merge_requests_approval_rules table.

MR acceptance checklist

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Related to #517963 (closed)