Convert Gradle build config to Kotlin

[aSemy]

Converts the build files to use the Kotlin DSL

Additionally

• bumps Gradle to 7.6
• enables Gradle Build & Configuration cache (improved build speeds, on CI & also when switching branches locally)
• uses centralised repo definitions https://docs.gradle.org/7.6/userguide/declaring_repositories.html#sub:centralized-repository-declaration
• Sets up Java Toolchain https://docs.gradle.org/7.6/userguide/toolchains.html

⚠️ Note that the git history of the build.gradle and settings.gradle files might be lost! It's difficult to get Git to recognise that a file has changed name and the content has also significantly changed. You might want to repeat the two commits a56e77e 7bc7f0c on the master branch if you want to keep the Git history

[aSemy]

hey @qoomon, I see you've approved this PR, when did you want to merge it? Is there something more that I can do?

[qoomon]

hi @aSemy sorry I was on a long vacation. I'll probably merge it between the holidays.

[aSemy]

@qoomon No problem, take your time :) Let me know if I can help or adjust anything.

[tacascer]

Hi @qoomon @aSemy I've stumbled across this project and saw that this PR has been open for a while.

I don't really know how to introduce myself except that I like this plugin and would like to help improve it :)

Anything I can do to help get this PR across the finish line?