Running DDL EXECUTE in function is incorrectly replicated to workers

Small reproducible example for what we encountered in a more complex case. Reproduces on PG15 + Citus 11.1 with at least one worker (besides the coordinator).

```
create table t1 (a int, b int);
select create_distributed_table('t1', 'a');

CREATE OR REPLACE FUNCTION exec_func()
RETURNS void
LANGUAGE plpgsql
AS $function$
BEGIN
EXECUTE $$
create index on t1 (a);
create index on t1 (b);
$$;
END
$function$
;

select exec_func();
```

Error:

```
SQL Error [42601]: ERROR: cannot insert multiple commands into a prepared statement
Where: while executing command on worker:5432
SQL statement "

create index on t1 (a);

create index on t1 (b);

"
PL/pgSQL function exec_func() line 3 at EXECUTE
```

Creating one index inside the `EXECUTE` works fine. It fails when creating multiple ones. Seems like it's trying to replicate things inside `EXECUTE` in a prepared statement to the workers, but since it contains two statements this fails?