Legacy Libraries Deprecation: Warnings in the UI

[kdmccormick]

Background

Verawood will be the final release to support Legacy Library viewing and editing. See the parent epic for the timeline, the DEPR ticket for the full details.

We might want to add more warnings into the UI so that this doesn't take authors by surprise.

Existing warnings

On the libraries list

On the legacy libraries list

Possible warnings we could add

When creating a legacy library (Draft PR: #2563)

When editing a legacy library

Warning could go between the title and the editor

When linking a legacy library

Warning could be added at the top of the Randomized Content Block editor

[jmakowski1123]

Instead of "future release", I think it's certain enough that we can specify Willow. Lmk if you disagree.