`on_stop` is not automatically triggered with pipelines for merge requests

What does this MR do?

This bug happens only if users use Pipelines for merge requests.

Currently, review app cannot be automatically terminated (see https://gitlab.com/gitlab-org/gitlab-ce/issues/60885#note_163759744) after the source branch is deleted. This is because deployments.ref stored merge request ref path instead of branch ref, thus StopEnvironmentsService cannot correctly lookup environments.

We fix this problem by explicitly hooking merge request close/merge event, which means the clear intention that users will no longer need review app to be alive.

close https://gitlab.com/gitlab-org/gitlab-ce/issues/60885

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team