Enable semi-linear merge trains (behind feature flag)

What does this MR do and why?

This expands the feature flag fast_forward_merge_trains_support to also include semi-linear merge trains.

• Feature issue: #26996 (closed)
• Rollout issue: #282442 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Enable the following feature flag:

   Feature.enable(:fast_forward_merge_trains_support)

2. Seed a project with a bunch of merge requests:

   bin/rake gitlab:seed:merge_trains:project

3. Visit the project, go to Settings > Merge Requests and change the Merge method to Semi-linear.
4. Visit a few of the open merge requests in separate tabs or windows
5. Wait for the pipeline to finish on all of them (wait for the button Set to auto-merge to change to Merge)
6. In rapid succession, press Merge for each of them to start a Semi-linear merge train
7. Navigate to Build > Pipelines in the sidebar
8. Verify that the commits for the merge trains look as expected.
9. Verify that all of the merge trains get merged and that the train does not get re-created at any point

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.