[Feature Request] Support Feature Flag Archival

[jed326]

Is your feature request related to a problem? Please describe

Today there is no concept of feature flag archival. This means that when a feature flag is removed, existing clusters that are using the feature flag will run into problems when trying to upgrade to the new software version without the feature flag.

See: #12074 (comment)

Describe the solution you'd like

We should have something like 68d07fc to support archival of old feature flags. Moreover, we should print some sort of deprecation message when old feature flags are found.

Related component

Other

Describe alternatives you've considered

No response

Additional context

No response

[jed326]

Some more things to think about

1. What would we do if we need to change the name of an experimental feature?
2. How add a test to cover this?
3. Can we generalize this to all static settings instead of just feature flags?

[peternied]

[Triage - attendees 1 2 3 4]
@jed326 Thanks for filing, looking forward to PRs that address this

[jed326]

Example of renaming feature flag: #13799