Use Kotlin 1.6 for API and language version in reference documentation

[philwebb]

KotlinConventions currently uses 1.3, we should upgrade.

[wilkinsona]

I think we should only do this in the docs, otherwise we’re raising our Kotlin baseline from 1.3 to 1.6. That feels a bit too much for 2.7.

[philwebb]

@wilkinsona What's the difference between the version in the conventions and the version in the gradle.properties. I assumed we were already on 1.6 and had just missed it in the conventions.

[wilkinsona]

Our Kotlin setup is a bit like compiling with Java 11 and using -release 8 to retain compatibility with an earlier version. While we depend on Kotlin 1.6, setting the API and language version to 1.3 produces Kotlin 1.3 compatible byte code.