pgx/v4/stdlib was only used in test infrastructure, but its transitive
dependency on jackc/pgproto3/v2 exposed the build to GO-2026-4518 (DoS
via negative field length panic in DataRow.Decode).
Replaces the pgconn.ParseConfig call via GenerateDSN with url.Parse for
DSN validation, and SELECT current_database() for retrieving the
database name, removing the last non-test dependency on the pgx/v4.
Updates TestUInt64Overflow in tests/postgres to match pgx/v5 error
format: v5 renamed Int to Int64 across pgtype to match database/sql
conventions, so the overflow error now says "int64" instead of "Int8".
Bumps minimum Go version to 1.24 (required by pgx/v5).
ea8a35d497