Move build scripts to Kotlin

[martinbonnin]

This is a follow up from #398

This PR converts build scripts to Kotlin. I tried as much as possible to preserve the existing behaviour even if it's not really idiomatic at the moment.
This PR also bumps the Gradle version to 6.9. This is the last version that ships with the "maven" plugin and still supports Kotlin DSL.

Follow ups:

• Migrate "maven" to "maven-publish"
• Make the build more idiomatic

[elefeint]

@jamesward Would you like to help review this PR?

[vlsi]

@martinbonnin , you might want to add an intermediate commit that renames build.gradle -> build.gradle.kts to help Git for displaying the history: https://stackoverflow.com/questions/44310285/converting-file-tracked-with-git-from-java-to-kotlin-in-android-studio/44320452#44320452

[martinbonnin]

you might want to add an intermediate commit that renames build.gradle -> build.gradle.kts

Good call 👍 . I just did. Also makes it easier to review the diff with the Groovy version from that commit

[elefeint]

I apologize for the delays.
Could you rebase this off latest?

[martinbonnin]

Could you rebase this off latest?

It should be good now

[elefeint]

Thank you!

[vlsi]

This should have been "v\$version"

[elefeint]

In #437

[vlsi]

This should have been project.version rather than version from a jar task (there's a version property in jar task)

[elefeint]

I'll add it to #437; thanks!

[elefeint]

I verified that releasing process still works and the staged artifacts look good (I've dropped them, so no new release right now). Thanks, everyone!

[vlsi]

@elefeint , I see you squashed the commits before merging. It causes broken history since Git can't detect that build.gradle -> build.gradle.kts was rename+modifications.

The intermediate commit was vital for proper git history, see my comment in #423 (comment).

Would you please clarify if "rebase and merge" (with or without merge commit) was an option at all?