Resolve "[Feature flag] Rollout of `use_new_rule_finalize_approach`"

What does this MR do and why?

This feature flag has been turned on for nearly 3 weeks now.

By removing this feature flag, we save more information at the time of merge in regard to the approval state, this allows us to maintain the approval state post merge even if changes to the parent rules change.

To test:

• Create a project rule with a rule that requires 0 approvals
• Tick settings -> merge_requests -> approval settings -> Prevent editing approval rules in merge requests
• Create MR
• Approve
• Merge
• Change the project rule to require 1
• Ensure the MR still contains the rule as being optional

MR acceptance checklist

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Related to #427612 (closed)