sql: Fix UX for dropping a user with default privileges

[RichardJCai]

Example in 21.2

CREATE USER foo;
GRANT SELECT ON DATABASE defaultdb TO foo;
REVOKE ALL ON DATABASE defaultdb FROM foo;
DROP USER foo;

Results in error

Query 1 ERROR: ERROR: role foo cannot be dropped because some objects depend on it
privileges for default privileges on new relations for all roles
HINT: use SHOW DEFAULT PRIVILEGES FOR ROLE to find existing default privileges and execute ALTER DEFAULT PRIVILEGES {FOR ROLE ... / FOR ALL ROLES} REVOKE ... ON ... FROM ... to remove them
see: SHOW DEFAULT PRIVILEGES and ALTER DEFAULT PRIVILEGES

We should make it clear how to revoke that privilege, for example it is not clear which database it is even on.

[rafiss]

fixed by #77016

[georgeli-roblox]

it would be good to have a feature drop user <username> cascade so it will take care of all the clean-ups it needs on the DB side ? Thanks

[RichardJCai]

it would be good to have a feature drop user <username> cascade so it will take care of all the clean-ups it needs on the DB side ? Thanks

We plan on adding support for drop owned by (postgres has this instead of cascade)
#55381