Che should have a mechanism to inform user on removed/deleted plugins

Is your enhancement related to a problem? Please describe.

Currently, the Che plugin registry hosts every plugin and version published since its inception. However, in the future, we may want to be able to remove plugins from later releases without obtusely breaking old workspaces (e.g. https://github.com/eclipse/che-plugin-registry/pull/264).

Describe the solution you’d like

The plugin registry and Che server should be able to surface when a previously existing plugin has been removed from the registry. E.g. the plugin registry could, instead of responding 404 as though the plugin never existed, report info to the user with a possible path to fixing the problem. For old versions (e.g. the rc versions of che-theia), a message could be returned suggesting a move to a new version in their devfile. For completely deprecated plugins such as node-debug, it could at least explain why support was dropped.

Describe alternatives you’ve considered

With the current approach, removal of any plugin from the registry means the potential for non-startable workspaces when updating Che.

[l0rd]

This looks complicated for an issue that should happen seldom. It should not be a common case because in the devfile registry we always reference latest of plugins and we rarely remove a full plugin.

So an alternative approach would be:

1. We agree that we are going to deprecate plugin X
2. We remove any reference to plugin X in the devfile registry => sprint Y
3. We remove the plugin from the plugin-registry => sprint Y+3

[ericwill]

A cull of old plugins in the plugin registry is definitely warranted. 👍