Upgrade to Graylog 6

Graylog 6 is here https://graylog.org/post/announcing-graylog-6-0/  that means new `graylog-plugin-web-parent` is here.

We should upgrade to `6.x.x` as the plugin might not work correctly. I am not sure if there will be any breaking changes.

I will implement this in upcoming month/s + release new major version `3.0.0`.