Actually enable improved blocked process detection (#6426)

In #6405 I added better improved blocked process detection for isolation
tests. But when cleaning up unnecessary code I cleaned up a bit too
much. This actually includes the new function definition in our
migrations.

src/backend/distributed/sql/citus--11.1-1--11.2-1.sql

@@ -1 +1,2 @@
 #include "udfs/get_rebalance_progress/11.2-1.sql"
+#include "udfs/citus_isolation_test_session_is_blocked/11.2-1.sql"

src/backend/distributed/sql/downgrades/citus--11.2-1--11.1-1.sql

@@ -1 +1,2 @@
 #include "../udfs/get_rebalance_progress/11.1-1.sql"
+#include "../udfs/citus_isolation_test_session_is_blocked/11.1-1.sql"