Re-consider Java version compatibility

[EdricChan03]

This should probably be a discussion, but until now it'll be used to discuss as to what versions of Java should this project support.

Overview

Currently, this project requires a minimum version of Java 8, which is the earliest (still) supported Long-Term Support (LTS) release for Java.

However, the Gradle plugin that is used to generate API docs (Dokkatoo) expects Java 11, the 2nd earliest supported LTS. I suspect this is because there is a task present that uses the HttpClient class from the java.net package, which was first introduced in Java 11 - see adamko-dev/dokkatoo#13 for more info.

As such, we should probably consider as to what is the minimum version of Java that this project should support...

Suggested solutions

1. Bump the minimum version of Java to 11, the version that Dokkatoo expects. - This may prevent potential consumers from using the project though, especially if they are stuck at Java 10 or lower

2. Migrate back to using Kotlin's Dokka plugin. This is what Dokkatoo aims to achieve at some point anyways, which is essentially a rewrite of Dokka to use more Gradle-idiomatic features, as mentioned in the readme:

   Dokkatoo has a number of improvements over the existing Dokka Gradle Plugin:

   • Compatible with Gradle Build Cache
   • Compatible with Gradle Configuration Cache
   • Safe cross-project sharing and aggregation
   • Parallel execution