Update Issue Incidents to allow milestones to be used

What does this MR do?

A small MVC #297007 (closed) where we allow Category:Incident Management Incidents to use the milestone feature both in quick actions and the sidebar. We do not allow iterations to be used currently.

Testing

1. Navigate to Issues > List
2. Click "New Issue"
3. In the Issue type dropdown, select Incident
4. Check in the sidebar that milestones are available
5. Check that /milestone and /remove_milestone quick actions are available from the comment box
6. Check that merge requests still have milestones but not iterations

Screenshots (strongly suggested)

Before	After

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

Related to #297007 (closed)