postgresql_15: 15.6 -> 15.7, fix CVE-2024-4317

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

pkgs/servers/sql/postgresql/15.nix

···
 import ./generic.nix {
-  version = "15.6";
-  hash = "sha256-hFUUbtnGnJOlfelUrq0DAsr60DXCskIXXWqh4X68svs=";
+  version = "15.7";
+  hash = "sha256-pG/klIWrY4Xjnau7tlT10wSSBvds1pXiJCaHKVIJmPc=";
 }