Remove the support of 23.05 and 23.11

This is because SNM now supports the new sieve nixpkgs interface,
which is not backward compatible with previous releases.

flake.lock

@@ -47,43 +47,11 @@
         "type": "indirect"
       }
     },
-    "nixpkgs-23_05": {
-      "locked": {
-        "lastModified": 1704290814,
-        "narHash": "sha256-LWvKHp7kGxk/GEtlrGYV68qIvPHkU9iToomNFGagixU=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "70bdadeb94ffc8806c0570eb5c2695ad29f0e421",
-        "type": "github"
-      },
-      "original": {
-        "id": "nixpkgs",
-        "ref": "nixos-23.05",
-        "type": "indirect"
-      }
-    },
-    "nixpkgs-23_11": {
-      "locked": {
-        "lastModified": 1709884566,
-        "narHash": "sha256-NSYJg2sfdO/XS3L8XN/59Zhzn0dqWm7XtVnKI2mHq3w=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "2be119add7b37dc535da2dd4cba68e2cf8d1517e",
-        "type": "github"
-      },
-      "original": {
-        "id": "nixpkgs",
-        "ref": "nixos-23.11",
-        "type": "indirect"
-      }
-    },
     "root": {
       "inputs": {
         "blobs": "blobs",
         "flake-compat": "flake-compat",
         "nixpkgs": "nixpkgs",
-        "nixpkgs-23_05": "nixpkgs-23_05",
-        "nixpkgs-23_11": "nixpkgs-23_11",
         "utils": "utils"
       }
     },

flake.nix

@@ -8,15 +8,13 @@
     };
     utils.url = "github:numtide/flake-utils";
     nixpkgs.url = "flake:nixpkgs/nixos-unstable";
-    nixpkgs-23_05.url = "flake:nixpkgs/nixos-23.05";
-    nixpkgs-23_11.url = "flake:nixpkgs/nixos-23.11";
     blobs = {
       url = "gitlab:simple-nixos-mailserver/blobs";
       flake = false;
     };
   };
 
-  outputs = { self, utils, blobs, nixpkgs, nixpkgs-23_05, nixpkgs-23_11, ... }: let
+  outputs = { self, utils, blobs, nixpkgs, ... }: let
     lib = nixpkgs.lib;
     system = "x86_64-linux";
     pkgs = nixpkgs.legacyPackages.${system};
@@ -25,14 +23,6 @@
         name = "unstable";
         pkgs = nixpkgs.legacyPackages.${system};
       }
-      {
-        name = "23.05";
-        pkgs = nixpkgs-23_05.legacyPackages.${system};
-      }
-      {
-        name = "23.11";
-        pkgs = nixpkgs-23_11.legacyPackages.${system};
-      }
     ];
     testNames = [
       "internal"