Retry 5xx errors in federation client (matrix-org#9567)

Fixes matrix-org#8915

changelog.d/9567.bugfix

@@ -0,0 +1 @@
+Fix bug where federation requests were not correctly retried on 5xx responses.

synapse/http/matrixfederationclient.py

@@ -534,9 +534,10 @@ async def _send_request(
                             response.code, response_phrase, body
                         )
 
-                        # Retry if the error is a 429 (Too Many Requests),
-                        # otherwise just raise a standard HttpResponseException
-                        if response.code == 429:
+                        # Retry if the error is a 5xx or a 429 (Too Many
+                        # Requests), otherwise just raise a standard
+                        # `HttpResponseException`
+                        if 500 <= response.code < 600 or response.code == 429:
                             raise RequestSendFailed(exc, can_retry=True) from exc
                         else:
                             raise exc