Merge branch '298541-rearrange-merge-request-train-method' into 'master'

Move MergeRequest#train to EE::MergeRequest#merge_train

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/125773



Merged-by: Shinya Maeda <shinya@gitlab.com>
Approved-by: Shinya Maeda <shinya@gitlab.com>
Approved-by: Tianwen Chen <tchen@gitlab.com>
Reviewed-by: Tianwen Chen <tchen@gitlab.com>
Co-authored-by: drew <dscimino@gmail.com>