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-socket-emacs.socket

@@ -0,0 +1,12 @@
+[Install]
+WantedBy=sockets.target
+
+[Socket]
+DirectoryMode=0700
+FileDescriptorName=server
+ListenStream=%t/emacs/server
+SocketMode=0600
+
+[Unit]
+Description=Emacs text editor
+Documentation=info:emacs man:emacs(1) https://gnu.org/software/emacs/