Add deprecated warning for DISABLE_GRAVATAR and ENABLE_FEDERATED_AVATAR (#22324)

Backport #22318

Author: kdumontnu

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -735,9 +735,9 @@ and
 
 - `GRAVATAR_SOURCE`: **gravatar**: Can be `gravatar`, `duoshuo` or anything like
    `http://cn.gravatar.com/avatar/`.
-- `DISABLE_GRAVATAR`: **false**: Enable this to use local avatars only.
+- `DISABLE_GRAVATAR`: **false**: Enable this to use local avatars only. **DEPRECATED [v1.18+]** moved to database. Use admin panel to configure.
 - `ENABLE_FEDERATED_AVATAR`: **false**: Enable support for federated avatars (see
-   [http://www.libravatar.org](http://www.libravatar.org)).
+   [http://www.libravatar.org](http://www.libravatar.org)). **DEPRECATED [v1.18+]** moved to database. Use admin panel to configure.
 
 - `AVATAR_STORAGE_TYPE`: **default**: Storage type defined in `[storage.xxx]`. Default is `default` which will read `[storage]` if no section `[storage]` will be a type `local`.
 - `AVATAR_UPLOAD_PATH`: **data/avatars**: Path to store user avatar image files.