Add 20.09 Hydra jobset and remove 19.09

.hydra/declarative-jobsets.nix

@@ -67,8 +67,8 @@ let
         };
       };
     };
-    "nixos-19.09" = {
-      description = "Build the nixos-19.09 branch of Simple NixOS MailServer";
+    "nixos-20.09" = {
+      description = "Build the nixos-20.09 branch of Simple NixOS MailServer";
       checkinterval = "60";
       enabled = "1";
       nixexprinput = "snm";
@@ -81,7 +81,7 @@ let
       type = 0;
       inputs = {
         snm = {
-          value = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver nixos-19.09";
+          value = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver nixos-20.09";
           type = "git";
           emailresponsible = false;
         };