Add v2 approval rule associations to project group and mr

What does this MR do and why?

Adds new approval rule associations to project group and merge request for new approval rule architecture.

References

Follows Add v2 approval rule table and model (!179839 - merged)
Issue: https://gitlab.com/groups/gitlab-org/-/issues/512606
Epic: Rearchitecting Approval Rules (&12955)
POC: Draft: [ci skip][DO NOT MERGE][PoC] Start of Po... (!171114 - closed)

MR acceptance checklist

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Related to #512606 (closed)