message: use ExecStart with stdout >> redirection instead of StandardOutput which does not support append until systemd 240+
diff: @@ -9,7 +9,8 @@ After=network-online.target
  [Service]
  Type=simple
  ExecStartPre=/etc/casper/systemd_pre_start.sh
  -ExecStart=/usr/bin/casper-node validator /etc/casper/config.toml
  +#StandardOutput can only append log files from systemd version 240 + (Ubuntu 20.04). Use ExecStart with redirection as workaround.
  +ExecStart=/bin/sh -c 'exec /usr/bin/casper-node validator /etc/casper/config.toml >> /var/log/casper/casper-node.log'
  Restart=no
  User=casper
  Group=casper
  