Resolve ""Include merge request description" or any custom merge commit message not working with Merge Trains enabled"

What does this MR do?

The issue is that the merge_request.merge_params is:

=> {"force_remove_source_branch"=>"1",
 "auto_merge_strategy"=>"merge_train",
 "should_remove_source_branch"=>true,
 "commit_message"=>"Merge branch 'root-master-patch-28951' into 'master'\n\nUpdate asdfadsfasdf\n\nSee merge request flightjs/flight!782",
 "sha"=>"333bc3a0843c4cf304721e245d2ce73365303c71"}

where as the code needs it to be a symbol, otherwise it uses the default message

We use the merge_params here: https://gitlab.com/gitlab-org/gitlab/-/blob/f8fe86e5ca84a28c90b1dd7a4311a2c82c267e9a/ee/app/services/merge_trains/refresh_merge_request_service.rb#L65

This calls the commit message function here: https://gitlab.com/gitlab-org/gitlab/-/blob/f8fe86e5ca84a28c90b1dd7a4311a2c82c267e9a/app/services/merge_requests/merge_service.rb#L93

And the commit message function is here: https://gitlab.com/gitlab-org/gitlab/-/blob/f8fe86e5ca84a28c90b1dd7a4311a2c82c267e9a/app/services/merge_requests/merge_base_service.rb#L58

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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 #232957 (closed)