Secure workers use a dedicated queue

What does this MR do?

Asynchronous workers maintained by the Secure Group are assigned their own sidekiq queue: security_scans. Current Secure workers have been moved from the pipeline_default queue to the security_scans queue. The queue has a medium priority, which is lower than the previous queue's high priority. Evidence to justify this priority change is detailed in a comment below.

This work was requested in a MR for another piece of worker that contained a Secure worker !23669 (comment 280383086).

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

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