Fix notebook URL opened in browser when redirect file not used

With `notebook~=7.0` ServerApp is responsible for launching browser, not
NotebookApp anymore. In `notebook~=6.0`, `launch_browser` will append
notebook path for opening in browser even when `use_redirect_file` is
set to `false` - unlike ServerApp, ever since 60c66b6.

jupyter_server/serverapp.py

@@ -2781,6 +2781,10 @@ def _prepare_browser_open(self):
             if self.identity_provider.token:
                 uri = url_concat(uri, {"token": self.identity_provider.token})
 
+            if self.file_to_run:
+                file_to_run_relpath = self._resolve_file_to_run_and_root_dir()
+                uri = url_escape(url_path_join(uri, *file_to_run_relpath.split(os.sep)))
+
         if self.file_to_run:  # noqa
             # Create a separate, temporary open-browser-file
             # pointing at a specific file.