Prevent modification of MR-level approval rules after MR is merged

Summary

Once an MR has been merged, a user can modify the MR-level approval rules, leaving the MR in a state where the approval rules differ to the rules at the time of merging.

Steps to reproduce

1. Create an MR with an MR-level approval rule e.g. requires 1 approvals
2. Satisfy rule and merge MR
3. After merge, edit MR and change number of approvals required
4. Save MR

Example Project

jay_mccure/test2target!119 (merged)

What is the current bug behavior?

Can change rules after merging

What is the expected correct behavior?

Cannot change rules after merging

Relevant logs and/or screenshots

MR requires 1 approval:

MR approved:

MR merged:

Edit MR to require 20 approvals:

Possible fixes