Draft: Resolve "Cannot remove "incident" label from incident issues"

What does this MR do?

🚧

Stop populating the ~incident label for incident issues.

Previously, we've used this special label to distinguish incidents from normal issues. Since we've introduced issue_type=incident this is not needed anymore.

TODO

• Make sure that the incident label is not added when
  • Manual incident issue creation
  • When updating the issue
  • Automatic incident issue (via alerts)
• Fix spec failures (remove incident label assertions)
• Do we need to tweak the usage ping (which relies on the incident label) as well?
• Possibly remove logic from IncidentManagement::CreateIncidentLabelService including LABEL_PROPERTIES

Screenshots (strongly suggested)

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 #277318 (closed)