Add a deprecated indication to plugins

From #2992 (closed) we've identified that hidden: true only hides a connector from the UI.

We should add a deprecated boolean flag and optionally a deprecation_warning string field for all plugins and extractor/loader variants.

Alternative would be:

deprecated:
  status: true #default is fales
  warning: "This was removed 2021-10-11 and replaced with X" # This would be optional and would have a sane default.

I like the idea of keeping hidden as it currently is. For connectors that we want to deprecate, we could take a staged process to remove it from the UI via hidden and then fully deprecate in a subsequent release.