Feat: adjust fields for ECS compatibility (#22)

Previously plugin was setting the top level sequence and host fields for every event (these aren't ECS compliant).

Also we're no longer overriding the host field if it's present in the generator line (after decoding by the codec).

set default_codec doc attribute

[skip ci] update gemspec summary

Version bump

For elastic/logstash#7993

[ci skip]

bump patch level for doc generation

dependency logstash-core-plugin-api >= 1.60 <= 2.99

Make sure all the gems were published under jruby

version 3.0.0

dependency logstash-core-plugin-api ~> 1.0

dependency logstash-core > 2.0.0 < 6.0.0.alpha1