bump scl submodule to v2.3.0

Among other changes this drops support for syslog-ng 3.2 and adds support for syslog-ng 3.4.
update readme

bump syslog-ng version to 3.4 and document new logpaths offered by the module
thanks to bumping the upstream scl config.
Merge branch 'feature/syslog-ng-3.4-bump' into develop