collab: Make admin column non-nullable on users table (#36307)

This PR updates the `admin` column on the `users` table to be
non-nullable.

We were already treating it like this in practice.

All rows in the production database already have a value for the `admin`
column.

Release Notes:

- N/A

crates/collab/migrations/20250816124707_make_admin_required_on_users.sql

@@ -0,0 +1,2 @@
+alter table users
+alter column admin set not null;