Update syslog-ng.conf from Ubuntu 20.04

[charlescng]

Fixes #589

Applied the diffs between syslog-ng.conf and /etc/syslog-ng/syslog-ng.conf installed by syslog-ng 3.25.1-3.

This resolves the following warnings:

WARNING: Configuration file format is too old, syslog-ng is running in compatibility mode. Please update it to use the syslog-ng 3.25 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; config-version='3.13'

WARNING: With use-dns(no), dns-cache() will be forced to 'no' too!;

WARNING: log-fifo-size() works differently starting with syslog-ng 3.22 to avoid dropping flow-controlled messages when log-fifo-size() is misconfigured. From now on, log-fifo-size() only affects messages that are not flow-controlled. (Flow-controlled log paths have the flags(flow-control) option set.) To enable the new behaviour, update the @Version string in your configuration and consider lowering the value of log-fifo-size().;

[Theaxiom]

LGTM! Thanks for your contribution! 👍