Do not require manual rebase for semi-linear and fast-forward merge trains

Release notes

Problem to solve

Currently, semi-linear and fast-forward merge trains require a manual rebase prior to adding to the train when the target branch is ahead of the source branch. While this is technically on-par with those merge methods when merge trains are disabled, it is an arbitrary limitation because merge trains can already auto-rebase.

Proposal

Merge trains support automatic rebases internally. In the first iteration, we can remove this limitation from the mergeability check.

A downside of this is that the mergeability check would not perform a rebase internally, so rebase conflicts would not be caught until asynchronously after the train ref fails to be created (see #421025 for more information about how the mergeability check could be updated to support rebases).

Intended users

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