JDK 14 builds and Gradle 6.3 upgrade implications

[Ismael Juma <is...@juma.me.uk>]

Hi all,

There have been 3 releases since Java 11, so we are halfway to the next
long-term support release (Java 17). It's a good time for Apache Kafka to
start testing with post Java 11 JDKs.

In order to do that, we need to upgrade to Gradle 6.3 which requires some
changes to how we run build commands that affect multiple Scala versions.
These are rarely used during development, but can be handy during releases.
For details, see:

https://github.com/apache/kafka/pull/7677

I have also added Jenkins PR and trunk jobs that run with Java 14. If your
PR is failing the Java 14 build, please make sure you rebase against master
to pick up:

https://github.com/apache/kafka/pull/8519

Let me know if you have any questions or if you have any issues.

Ismael