Use bytea rather than BLOB

synapse/storage/schema/delta/15/server_keys.sql

@@ -19,6 +19,6 @@ CREATE TABLE IF NOT EXISTS server_keys_json (
     from_server TEXT, -- Which server the keys were fetched from.
     ts_added_ms INTEGER, -- When the keys were fetched
     ts_valid_until_ms INTEGER, -- When this version of the keys exipires.
-    key_json BLOB, -- JSON certificate for the remote server.
+    key_json bytea, -- JSON certificate for the remote server.
     CONSTRAINT uniqueness UNIQUE (server_name, key_id, from_server)
 );