Update ExternalPullRequest on :synchronize action

What does this MR do?

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/67191

While testing the new "Pipeline for external pull request" feature on staging I realized that the feature was not working on branch push. While it was working correctly on pull request being opened and reopened.

The issue was caused by the fact that the sychronize webhook action was not been consumed on our side, which contained the source_sha change. That caused the ExternalPullRequest event to be outdated with an old source_sha which was always failing on the check pull_request.actual_branch_head?.

Screenshots

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