Fix fetching signing keys when `old_verify_keys` is omitted (#17568)

`old_verify_keys` isn't marked as required in
https://spec.matrix.org/v1.11/server-server-api/#get_matrixkeyv2server
and there's no functional difference between an empty object and
omitting the object, so I don't think there's any reason synapse should
explode when the field is omitted.

changelog.d/17568.bugfix

+Fix fetching federation signing keys from servers that omit `old_verify_keys`. Contributed by @tulir @ Beeper.

synapse/crypto/keyring.py

@@ -589,7 +589,7 @@ class BaseV2KeyFetcher(KeyFetcher):
                 % (server_name,)
             )
 
-        for key_id, key_data in response_json["old_verify_keys"].items():
+        for key_id, key_data in response_json.get("old_verify_keys", {}).items():
             if is_signing_algorithm_supported(key_id):
                 key_base64 = key_data["key"]
                 key_bytes = decode_base64(key_base64)