text: Title: Loudly deprecate `[Return]` setting
  Body: We added the `RETURN` statement to replace the `[Return]` setting in RF 5.0 (#4078). The reason was that the setting has various limitations explained in the aforementioned issue. At the moment the old setting still works and there's no visible deprecation warning. I believe a more loud deprecation should be added now for the following reasons:
  - It is confusing for users that there is both `RETURN` and `[Return]`. Properly deprecating and eventually removing the latter simplifies the syntax.
  - Syntax like this requires special handling internally in Robot. Being able to remove it simplifies the code.
  - It has been already over 1.5 years since RF 5.0 was released. Users have had time to start taking `RETURN` into use.
  - Although using `[Return]` will case a deprecation warning, it will still continue to work during the whole RF 7.x lifetime. It will earliest be removed in RF 8.0.
  
  Notice that the [Robotidy](https://robotidy.readthedocs.io/en/stable/index.html) tool can convert `[Return]` to `RETURN` automatically.