Fix merge train is not refreshed when the system aborts/drops a merge request

What does this MR do?

It seems when auto merge was aborted on a merge request, the system doesn't notify the event to the next merge request on the same train. This causes a problem that MRs are not properly updated in such situation.

We should fire the refresh event when MR is dropped(aborted) from a train.

Related: #13001 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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