The standard output value "syslog" was deprecated for a while and did fall
back to "journal". It causes a warning since systemd version 246 [1].
By removing the setting it will default to DefaultStandardOutput, which
defaults to "journal".
[1] https://github.com/systemd/systemd/blob/master/NEWS#L202Closesstrongswan/strongswan#181.
Use strongswan-starter for the legacy unit and simply strongswan for the
modern one (strongswan-swanctl is configured as alias, which should
cause the installation of symlinks when the service is enabled via
systemctl).