boot: Replace deprecated StandardOutput=syslog with journal, etc.

systemd deprecated this in v246.

Resolves: #2169
Signed-off-by: Simon McVittie <smcv@collabora.com>

src/boot/ostree-prepare-root.service

@@ -30,6 +30,6 @@ Before=initrd-root-fs.target
 Type=oneshot
 ExecStart=/usr/lib/ostree/ostree-prepare-root /sysroot
 StandardInput=null
-StandardOutput=syslog
-StandardError=syslog+console
+StandardOutput=journal
+StandardError=journal+console
 RemainAfterExit=yes

src/boot/ostree-remount.service

@@ -35,8 +35,8 @@ Type=oneshot
 RemainAfterExit=yes
 ExecStart=/usr/lib/ostree/ostree-remount
 StandardInput=null
-StandardOutput=syslog
-StandardError=syslog+console
+StandardOutput=journal
+StandardError=journal+console
 
 [Install]
 WantedBy=local-fs.target