Use standard merge commit for merge trains

What does this MR do and why?

This changes the merge commit for merge trains to align with what would get merged on the target branch. This is in preparation of enabling fast-forward merges and semi-linear merges from merge trains, as those will get fast-forward merged directly from the train ref.

The change is now enabled by default behind the feature flag standard_merge_train_ref_merge_commit, with rollout issue: #422724 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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