Feature flag: ask confirmation to upgrade instance to version where overridden feature flags (turned OFF) have been removed from code

Proposal

One of the steps around feature flags is that after the rollout has terminated, we remove the check from the code and the old code goes away.

But between that moment and the moment where the feature flag is default to true for the customer, one or more releases elapse.

While the feature flag is in the code, customers have the option to toggle it off for any particular reason, maybe the new code is breaking a flow or a feature in their specific pattern of usage.

This means that when they're upgrading GitLab to the version where the feature flag check was removed, the behavior will automatically, and probably unexpectedly, revert to the new code ("Feature Flag ON" scenario).

Context

Despite not being sure how prevalent and frequent this problem might be for our customers, we were wondering if it would be possible to:

1. Keep documented the milestone in which the feature flag was removed from the code.
2. When upgrading an instance, check the current feature flag settings for the instance 2.1 If any feature flag removed since last update is toggled OFF 2.2 Issue a (blocking?) warning, asking for confirmation to proceed anyway.