Fix update head pipeline process of Pipelines for merge requests

What does this MR do?

Currently, schedule_head_pipeline_update method doesn't take into account of Pipelines for merge requests (Because pipeline.ref doesn't match merge request's source branch name).

We fix this problem by using the dedicated all_merge_requests method.

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

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