Do not auto abort Merge Trains on FF projects

Summary

For projects with FF merges only, !17886 (merged) aborts auto-merge MRs when pushing changes to the target branch. It did not take into account that there are multiple auto-merge strategies.

What is the current bug behavior?

It does not check the auto_merge_strategy and it will cancel all types of auto merges:

• merge_when_pipeline_succeeds
• add_to_merge_train_when_pipeline_succeeds
• merge_train

What is the expected correct behavior?

It should cancel only:

• merge_when_pipeline_succeeds