Add `Incident` label to issues created by the Alert Bot

What does this MR do?

This MR adds the incident label to each incident issue from firing Prometheus alerts. The incident label will be created if missing:

• Name: incident
• Color: #CC0033
• Description: Denotes a disruption to IT services and the associated issues require immediate attention

~Documentation CE backport https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31107

Refs https://gitlab.com/gitlab-org/gitlab-ce/issues/63373

TODO

• Tweak policy for Alert bot to allow labeling issues without being a real project member. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14801
• Check if group labels are respected
• Wait for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14801

Screenshots

Incident with label
Label

Docs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31107

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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