Cancel review app deployment when MR is merged

What does this MR do?

Issue: #213684 (closed)

Automatically cancel review deploy jobs when a MR is merged. This resolves a race condition where stale review app deployments could be created that would never be cleaned up by the review app stop action.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Style guides

Availability and Testing

If there are side-effects in a review app deployment job those will get cancelled. I'm not sure how realistic this concern is.