Pass `isolation_level` to `runWithConnection` (#11847)

This was missed in https://github.com/matrix-org/synapse/pull/11799

changelog.d/11847.misc

@@ -0,0 +1 @@
+Preparation for reducing Postgres serialization errors: allow setting transaction isolation level. Contributed by Nick @ Beeper.

synapse/storage/database.py

@@ -748,6 +748,7 @@ class DatabasePool:
                     func,
                     *args,
                     db_autocommit=db_autocommit,
+                    isolation_level=isolation_level,
                     **kwargs,
                 )