Archive Labels: Change feature flag actor to group

What does this MR do and why?

This MR changes the feature flag actor to group instead of :instance for the labels_archive feature flag

For the rollout, we prefer the actor to be on group level, as discussed.

🛠️ with ❤️ at Siemens

References

• Archived labels (#4233)
• #554227 (comment 2868875532)

Screenshots or screen recordings

When the actor is enabled and some labels are archived, the labels are now filtered:

Archived and unarchived labels in auto-complete

Archived and unarchived labels in issue dropdown

How to set up and validate locally

1. Enable feature flag via rails c for a specific group:

Feature.enable(:labels_archive, group)

Archive / Unarchive labels

2. Archive a label: Go to Any project > Manage > Labels and archive a label in the three dot dropdown. It should now show up in the Archived tab.
3. Go to the Edit view of the archived label. Now, you should see the box on the top and should be able to unarchive it.

Auto-complete / issus dropdown

4. After archiving a label, you should no longer see it when using /label ~... or in the label dropdown in an issue. Auto-completions are cached for three minutes - you can reduce the cache to a second by editing app/controllers/concerns/autocomplete_sources/expires_in.rb and setting AUTOCOMPLETE_EXPIRES_IN = 1.second or by clearing the cache.

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Related to #4233