upgrade kafka client to 2.3.0 (#230)

* also move travis testing to openjdk on travis

Bump plugin version and changelog (#229)

[Backport to 7.x] Remove unnecessary sleep after exhausted retries (#218

)

* Remove unnecessary sleep after exhausted retries

Also fixes up some issues with logging, where the batch size was calculated
incorrectly.

Backport to 6.x of fixes from more recent branches (#219)


* Fixed incorrect millisecond to second conversion for retry_backoff_ms
* Improve error logging when failing to create producer
* pin bundler version to < 2
* Remove unnecessary sleep after exhausted retries

Changelog and version bump

Fixes #216

remove obsolete fields

Doc tidy up

Fixes #213

bump kafka-clients to 2.1.0

Update changelog, bump version

Also change license definition in gemspec to SPDX compliant text.

Fixes #209

fix documentation about kafka client versions