Creating a placeholder issue for SQLancer to run on Postgres v15.

Alike support for v14 Issue ( #1042 ) - I intend to let SQLancer run on the target version (here REL_15_STABLE) and indicate errors - and if any, should ideally be fixed one-by-one.

Major changes in Postgres v15:

• MERGE
• NULLS NOT DISTINCT
• public schema being private by default would be a good thing to watch out for (in tests)
• CREATE VIEW (security_invoker)