Fix inconsistent cancel merge train button label

This is an actionable insight from the UX Department: MRs experience async critique research (insight).

What

In the merge request widget, when using the merge trains feature, the “start” and “cancel” buttons have inconsistent language:

1. Before starting the merge train: Start merge train when pipeline succeeds
2. After setting the merge train: Cancel automatic merge

How

Change the button text from "Cancel automatic merge" to "Cancel".