Add new merge strategy in frontend as preferred

What does this MR do and why?

Related to #407352 (closed) and #8128 (closed).

This MR builds on top of !121828 (merged) and !121847 (merged).

It adds the new merge strategy to frontend and uses it as preferred over merge_when_pipeline_succeeds.

Screenshots or screen recordings

Various scenarios

Approval is given before pipeline succeeds (same behavior as the current MWPS)

merge_when_checks_pass_approve_before

Approval is given after pipeline succeeds (new behavior that merges the MR)

merge_when_checks_pass_after_approval

Merge is clicked after the approval is given and pipeline succeeds (current behavior without auto-merge)

merge_when_checks_pass_all_pass_before

With feature flag :merge_when_checks_pass disabled (uses MWPS)

merge_when_checks_pass_mwps

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:merge_when_checks_pass)

2. Open an MR which requires approval
3. Click "Set to auto-merge"
4. Wait until pipeline succeeds
5. Approve
6. MR gets merged

Todos

• Documentation page

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.