drop old 24.xx channels

CI is breaking

images/devenv/default.nix

@@ -3,7 +3,7 @@
 }:
 (docker-nixpkgs.nix.override {
   # only available since 24.05
-  extraContents = if devenv == null then [] else [ devenv ];
+  extraContents = [ devenv ];
 }).overrideAttrs (prev: {
   meta = (prev.meta or { }) // {
     description = "Nix and devenv image";