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-27-emacsclient.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Categories=Development;TextEditor;
+Comment=Edit text
+Exec=@emacs@/bin/emacsclient -c %F
+GenericName=Text Editor
+Icon=emacs
+Keywords=Text;Editor;
+MimeType=text/english;text/plain;text/x-makefile;text/x-c++hdr;text/x-c++src;text/x-chdr;text/x-csrc;text/x-java;text/x-moc;text/x-pascal;text/x-tcl;text/x-tex;application/x-shellscript;text/x-c;text/x-c++;
+Name=Emacs Client
+StartupWMClass=Emacs
+Terminal=false
+Type=Application