Cannot remove "incident" label from incident issues

Summary

The Incident label cannot be removed from an issue once it has been added once.

Steps to reproduce

1. Add the incident label 1. Create an incident issue (manually via New Incident or automatically via Alert Management)
2. Try to remove it by clicking on the x close to the label under the labels section on the right side of the web UI
3. The label is marked as removed on the issue activity but effectively remains there

What is the current bug behavior?

Please see https://gitlab.gnome.org/Infrastructure/GitLab/-/issues/463 for the exact behavior.

Results of GitLab environment info

Expand for output related to GitLab environment info

gitlab_env.log

Results of GitLab application Check

Expand for output related to the GitLab application check

gitlab_check.log

Proposal for fixing

This behavior is in place because we currently lack an issue type filter for the issue list page - without the labels, you can't surface incidents in the issue list. That is being fixed as part of #268152 (closed). When that is completed, we can:

1. Allow people to remove the incident label
2. ~~STOP automatically applying the incident label to new incidents - this happens when incidents are created manually AND automatically by the GitLab Alert Bot.~~ We will do this after #268152 (closed) is completed, but we can do the first step now.

Technical proposal

Removing the caller of Issues::BaseServce#add_incident_label during update should stop (re-)adding the ~incident label.