Mark the StageUpdateWorker as idempotent

What does this MR do?

This worker was already idempotent, it updates the status of a stage based on the jobs inside it.

This also means we can safely deduplicate it.

Related to gitlab-com/gl-infra/scalability#285 (closed)

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