Remove all usage of :workflow_rules feature flag

What does this MR do?

This MR removes the workflow_rules feature flag, which has been running enabled on gitlab.com for two months and enabled by default for everyone for about a month. We're confident in the feature and ready to remove this flag.

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

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

The feature has been running on gitlab.com and enabled by default without issues, so this will close #29654 (closed)