fix a display bug in webhook notifications with unicode characters

see: https://github.com/ansible/awx/issues/7400

awx/main/notifications/webhook_backend.py

@@ -68,7 +68,7 @@ class WebhookBackend(AWXBaseEmailBackend, CustomNotificationBase):
                 auth = (self.username, self.password)
             r = chosen_method("{}".format(m.recipients()[0]),
                               auth=auth,
-                              json=m.body,
+                              data=json.dumps(m.body, ensure_ascii=False).encode('utf-8'),
                               headers=self.headers,
                               verify=(not self.disable_ssl_verification))
             if r.status_code >= 400: