Use the non-obsolete devshell (#67)

flake.nix

@@ -78,7 +78,8 @@
           program = "${self.packages.${system}.default}/bin/Gitea.Declarative";
         };
       };
-      devShell = pkgs.mkShell {
+      devShells = {
+        default = pkgs.mkShell {
           buildInputs = with pkgs; [
             (with dotnetCorePackages;
               combinePackages [
@@ -92,5 +93,6 @@
             pkgs.shellcheck
           ];
         };
+      };
     });
 }