etc: systemd services: switch to "journal" for output and error logging.

The "syslog" method has been deprecated for years, and issues a warning:

  Standard output type syslog is obsolete, automatically updating to
  journal. Please update your unit file, and consider removing the setting
  altogether.

Fixes: #48323

* etc/guix-daemon.service.in (StandardOutput): Use "journal"
(StandardError): Likewise.
* etc/guix-publish.service.in (StandardOutput): Likewise.
(StandardError): Likewise.

etc/guix-daemon.service.in

@@ -9,8 +9,8 @@ Description=Build daemon for GNU Guix
 ExecStart=@localstatedir@/guix/profiles/per-user/root/current-guix/bin/guix-daemon \
     --build-users-group=guixbuild --discover=no
 Environment='GUIX_LOCPATH=@localstatedir@/guix/profiles/per-user/root/guix-profile/lib/locale' LC_ALL=en_US.utf8
-StandardOutput=syslog
-StandardError=syslog
+StandardOutput=journal
+StandardError=journal
 
 # Work around a nasty systemd ‘feature’ that kills the entire process tree
 # (including the daemon!) if any child, such as cc1plus, runs out of memory.