## Problem
We currently can't create subscriptions in PG14 and PG15 because only
superusers can, and PG16 requires adding roles to
pg_create_subscription.
## Summary of changes
I added changes to PG14 and PG15 that allow neon_superuser to bypass the
superuser requirement. For PG16, I didn't do that but added a migration
that adds neon_superuser to pg_create_subscription. Also added a test to
make sure it works.