Fix race condition issue on MergeRequests::CreateService

What does this MR do?

The service class Security::SyncReportsToApprovalRulesService which runs on Sidekiq depends on the head_pipeline attribute of merge requests to be set. But the head_pipeline attribute of merge requests are set on background job which can be finished before or after we run the logic of Security::SyncReportsToApprovalRulesService.

This MR fixes this race condition issue by scheduling the Security::SyncReportsToApprovalRulesService after we set the head_pipeline attribute of merge requests.

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