Upgrade to Gradle 6.0 to simplify build logic

[aesteve]

Hello,

With Gradle 6.0 being released, we now can benefit from withJavadocJar, withSourcesJar.

This should help removing stuff here.

Never tried it so far, so I won't fancy a PR, but if you experiment with it, it'd also help vertx-aws-sdk (in Reactiverse, too). Please ping me if you do so :)

Thank you.

[oliver-brm]

This is rather old - in the meantime Gradle 7 is in use. I think this can be closed.