Notify new merge Request conflicts only once

What does this MR do?

This MR fixes excessive merge request conflict notification issues from the reverted MR !27870 (merged)

It seems like it was caused by the fact that merge_request.mark_as_unchecked is called in app/services/merge_requests/update_service.rb, app/services/merge_requests/refresh_service.rb, and app/services/merge_requests/reopen_service.rb which will change the MR's state from cannot_be_merged to cannot_be_merged_recheck.

Then the subsequent mergeability check on pageload of the merge request will trigger async check which means the state changes from cannot_be_merged_recheck to checking.

Which means we lost the track of the fact that it was recheck of cannot_be_merged and sent more emails than expected!

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #212886 (closed) #210500 (closed)