Pipeline must pass when manual job hasn't run

What does this MR do?

MR pipelines are showing a status of blocked rather than passed. This is caused by the manual job branch_release_job not having run. This job is optional and the pipeline should pass even if it doesn't run. This MR fixes that by setting allow_failure: true.

What are the relevant issue numbers?

NA

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer