Update syslog-ng config file format

I noticed the following message during reboot on my M50 (tn02) running TrueNAS-11.2-MASTER-201904190659

[2019-04-22T16:01:13.248185] WARNING: Configuration file format is too old, syslog-ng is running in compatibility mode. Please update it to use the syslog-ng 3.20 format at your time of convenience. To upgrade the configuration, please review the warnings about incompatible changes printed by syslog-ng, and once completed change the @version header at the top of the configuration file.;