api: Fix encoding of strings in display settings endpoint.

* Remove unnecessary json validator for string validator.
* Update frontend to pass right validator.
* Update zulip.yaml to pass right parameter for curl request
  in openapi.
* Update python_examples to pass right paramater.

Fixes part of #18035.

zerver/openapi/python_examples.py

@@ -1131,7 +1131,7 @@ def update_display_settings(client: Client) -> None:
     # Change emoji set used for display to Google modern.
     request = {
         "left_side_userlist": True,
-        "emojiset": '"google"',
+        "emojiset": "google",
     }
     result = client.call_endpoint("settings/display", method="PATCH", request=request)
     # {code_example|end}