Reduce urgency of EmailsOnPushWorker

What does this MR do?

Emails are not time-critical, so we can reduce the urgency of this worker.

See gitlab-com/gl-infra/scalability#264 (closed) for some more details.

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

Closes gitlab-com/gl-infra/scalability#264 (closed)