services/emacs: Update systemd definitions, drop Emacs 26 support

Emacs 27 added Type=notify support and updated the service definition to
remove the use of `emacsclient' to kill the service. Emacs 28 changes
the `StartupWMClass' in emacsclient.desktop to `Emacsd'. Update our
emacs.service and emacsclient.desktop definitions to match upstream
changes.

When killing emacs.service, the socket is removed, and subsequently
starting the service manually results in a service without a socket.
Prevent this by adding `RefuseManualStart=true' to the service's Unit
definition.

Drop Emacs 26 support as it is no longer shipped in nixpkgs. Update the
tests to verify the following configuration scenarios:

- Emacs version: 27, 28
- Socket activation: disabled, enabled

tests/modules/services/emacs/emacs-service-27.nix

@@ -0,0 +1,37 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    nixpkgs.overlays = [
+      (self: super: rec {
+        emacs = pkgs.writeShellScriptBin "dummy-emacs-27.2" "" // {
+          outPath = "@emacs@";
+        };
+        emacsPackagesFor = _:
+          makeScope super.newScope (_: { emacsWithPackages = _: emacs; });
+      })
+    ];
+
+    programs.emacs.enable = true;
+    services.emacs.enable = true;
+    services.emacs.client.enable = true;
+
+    nmt.script = ''
+      assertPathNotExists home-files/.config/systemd/user/emacs.socket
+      assertFileExists home-files/.config/systemd/user/emacs.service
+      assertFileExists home-path/share/applications/emacsclient.desktop
+
+      assertFileContent home-files/.config/systemd/user/emacs.service \
+                        ${
+                          pkgs.substituteAll {
+                            inherit (pkgs) runtimeShell;
+                            src = ./emacs-service-emacs.service;
+                          }
+                        }
+      assertFileContent home-path/share/applications/emacsclient.desktop \
+                        ${./emacs-27-emacsclient.desktop}
+    '';
+  };
+}