fix(falcon/async_resource): replace the use of removed resp.body

Author: vytas7

slack_bolt/adapter/falcon/async_resource.py

@@ -1,6 +1,7 @@
 from datetime import datetime
 from http import HTTPStatus
 
+from falcon import MEDIA_TEXT
 from falcon import version as falcon_version
 from falcon.asgi import Request, Response
 from slack_bolt import BoltResponse
@@ -40,9 +41,9 @@ async def on_get(self, req: Request, resp: Response):
                 await self._write_response(bolt_resp, resp)
                 return
 
-        resp.status = "404"
-        # Falcon 4.x w/ mypy fails to correctly infer the str type here
-        resp.body = "The page is not found..."
+        resp.status = HTTPStatus.NOT_FOUND
+        resp.content_type = MEDIA_TEXT
+        resp.text = "The page is not found..."
 
     async def on_post(self, req: Request, resp: Response):
         bolt_req = await self._to_bolt_request(req)