Stop testing stable nixos

We only test and support matching nixpkgs versions to simpliy alignment
with breaking changes on nixos unstable.

flake.lock

@@ -93,29 +93,12 @@
         "type": "github"
       }
     },
-    "nixpkgs-25_05": {
-      "locked": {
-        "lastModified": 1761999846,
-        "narHash": "sha256-IYlYnp4O4dzEpL77BD/lj5NnJy2J8qbHkNSFiPBCbqo=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "3de8f8d73e35724bf9abef41f1bdbedda1e14a31",
-        "type": "github"
-      },
-      "original": {
-        "owner": "NixOS",
-        "ref": "nixos-25.05",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "root": {
       "inputs": {
         "blobs": "blobs",
         "flake-compat": "flake-compat",
         "git-hooks": "git-hooks",
-        "nixpkgs": "nixpkgs",
-        "nixpkgs-25_05": "nixpkgs-25_05"
+        "nixpkgs": "nixpkgs"
       }
     }
   },

flake.nix

@@ -13,7 +13,6 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    nixpkgs-25_05.url = "github:NixOS/nixpkgs/nixos-25.05";
     blobs = {
       url = "gitlab:simple-nixos-mailserver/blobs";
       flake = false;
@@ -26,7 +25,6 @@
       blobs,
       git-hooks,
       nixpkgs,
-      nixpkgs-25_05,
       ...
     }:
     let
@@ -39,11 +37,6 @@
           nixpkgs = nixpkgs;
           pkgs = nixpkgs.legacyPackages.${system};
         }
-        {
-          name = "25.05";
-          nixpkgs = nixpkgs-25_05;
-          pkgs = nixpkgs-25_05.legacyPackages.${system};
-        }
       ];
       testNames = [
         "clamav"