Skip rebase check when using merge trains for detailed merge status

What does this MR do and why?

We fixed a bug with showing immediate merge for a MR using merge trains, but with a non merge commit merge strategy. This caused a change in the detailed merge status response. To maintain previous behaviour of this particular API response, we are going to change the behaviour back to as it was before the change. Note: This isn't reverting the fix.

Problem: When a project has merge trains enabled and requires fast-forward merges, merge requests would incorrectly show need_rebase as the detailed merge status, even though merge trains automatically rebase commits before merging.

Solution: Pass skip_rebase_check: true to execute_merge_checks when merge_trains_enabled? returns true for the project.

References

Related to !216871 (comment 3002911037)

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