flake: update from deprecated flake attributes

flake.nix

@@ -137,7 +137,8 @@
 
         defaultApp = apps.diff-flake;
 
-        devShell = pkgs.mkShell {
+        devShells = {
+          default = pkgs.mkShell {
             name = "NixOS-config";
 
             nativeBuildInputs = with pkgs; [
@@ -147,6 +148,7 @@
 
             inherit (self.checks.${system}.pre-commit) shellHook;
           };
+        };
 
         packages =
           let
@@ -157,8 +159,6 @@
           in
           finalPackages;
       }) // {
-      overlay = self.overlays.pkgs;
-
       overlays = import ./overlays // {
         lib = final: prev: { inherit lib; };
         pkgs = final: prev: {