Bump Kotlin version to 1.4 or 1.8

[szpak]

To keep compatibility with older Gradle versions, 1.2.0 enforces Kotlin 1.3 even with building Gradle 8.x. After the 1.2.0 release, the minimal required Gradle version could be bumped (7.x+?), so Kotlin 1.8 (or 1.4) would be more natural.

[TWiStErRob]

Obligatory related link: https://docs.gradle.org/current/userguide/compatibility.html

Bumping to 1.4 wouldn't give you any benefit really, the decision should be made whether you want to support Gradle 5.x and 6.x. Bumping Kotlin support is just a consequence of that decision.

Note that Gradle 8 is using Kotlin 1.8, and we override the language/api version to 1.3. Even with 1.4 we would get the warning:

w: Language version 1.4 is deprecated and its support will be removed in a future version of Kotlin

So the only thing would change in the code is 1.3 to 1.4 and the ability to drop Gradle 5.x and 6.x. And Gradle 7.x would still require the override, and this will continue forever unless you only support the latest Gradle version in each release.
This is perfectly fine though, as I mentioned somewhere already lowering the Kotlin version is pretty much the same as what we do with targetCompatibility = 1.8 with Java.

Additionally, I'm not sure dropping Gradle 6.x range would be welcome in the community, after all even this project was using 6.9 up until this week 😥

[vlsi]

I guess bumping to Gradle 6.1 enables using Shared Build Services, which might help a lot to stop relying on the root project.

[TWiStErRob]

Nice! Exactly what I meant by "the decision should be made whether you want to support Gradle 5.x and 6.x".

[szpak]

This task is in the milestone 2.0.0, but we've already bumped the minimal required Gradle version to 6.0 (or 6.2) in 2.0.0 and I wonder, if we shouldn't wait with dropping Gradle <7 support to 3.0.0 (possible to release somehow soon, if problematic for Gradle 9 - or 10 - compatibility).?

[TWiStErRob]

I think it's fine, I would wait for Kotlin 1.9 or 2.0, and until Gradle updates to those. Until then there's no real push, unless you really want to use Kotlin 1.9 features in this plugin right now :)

I'm curious if there will be any backlash from Gradle 6 minimum.

[TWiStErRob]

Hmm... gradle/gradle#25868

[TWiStErRob]

Unblocks #299