options: remove --focus-on-open and add --focus-on

replaces the old focus-on-open option with a more generic focus-on
options that can be extended.

adjust the only platform that uses that option.

Fixes #8337

DOCS/interface-changes.rst

@@ -63,6 +63,7 @@ Interface changes
     - add `--border-background` option
     - add `video-target-params` property
     - add `hdr10plus` sub-parameter to `format` video filter
+    - remove `--focus-on-open` and add replacement `--focus-on`
  --- mpv 0.37.0 ---
     - `--save-position-on-quit` and its associated commands now store state files
       in %LOCALAPPDATA% instead of %APPDATA% directory by default on Windows.