Use NixOS 21.05

configuration.nix

@@ -3,7 +3,7 @@ let
   secrets = import ./secrets.nix;
   hostname = "mail-dumpstack-io";
   domain = "dumpstack.io";
-  branch = "20.09";
+  branch = "21.05";
 in {
   imports = [
     ./hardware-configuration.nix
@@ -56,7 +56,7 @@ in {
   };
 
   # read release notes carefully before changing it
-  system.stateVersion = "20.09";
+  system.stateVersion = "21.05";
 
   nix = {
     optimise.automatic = true;