postgresql_12: 12.18 -> 12.19

Announcement: https://www.postgresql.org/about/news/postgresql-163-157-1412-1315-and-1219-released-2858/

pkgs/servers/sql/postgresql/12.nix

···
 import ./generic.nix {
-  version = "12.18";
-  hash = "sha256-T5kZcl2UHOmGjgf+HtHTqGdIWZtIM4ZUdYOSi3TDkYo=";
+  version = "12.19";
+  hash = "sha256-YX495Swi6CL09X0B1bIkBQPhmKnsyvWYqFEQm9GOb7s=";
 }