cannot upgrade table to a reference table in mx

- created a distributed table with a single shard (in mx)
- run upgrade_to_reference table

```sql
set citus.shard_count to 1;
create table m1(a int, b int);
select create_distributed_table('m1', 'a');

select upgrade_to_reference_table('m1');
ERROR: cannot upgrade to reference table
DETAIL: Upgrade is only supported for statement-based replicated tables but "m1" is streaming replicated
```