Update Kotlin to 2.0

[fzhinkin]

No description provided.

[whyoleg]

Just some minor comments :)

[lppedd]

Does this change retain compatibility with 1.9.x?

[fzhinkin]

@lppedd, For JVM, there are some forward compatibility guarantees: https://kotlinlang.org/docs/kotlin-evolution.html#evolving-the-binary-format; but for KMP, as per JetBrains/kotlin-web-site#4216, there are no such guarantees.
I've checked if a 1.9.x-based project will work with a 2.0-based kotlinx-io (0.4.0-alpha built from this branch and hosted in https://oss.sonatype.org/content/repositories/orgjetbrainskotlinx-3346) and it seems to work fine for all targets except Wasm.

UPD: for the Wasm, an issue could be fixed to some extent by restricting stdlib version on the consumer side so it remains 1.9.x (implementation(kotlin("stdlib")) { version { strictly("1.9.24") } }).

[lppedd]

@fzhinkin so basically even setting

apiVersion = KotlinVersion.KOTLIN_1_9
languageVersion = KotlinVersion.KOTLIN_1_9

wouldn't help? I'm just asking that because libraries suddenly requiring 2.0+ means whoever is using 1.9.x can't update anything. I've seen coroutines doing the same, serialization too. This seems a bit risky imo.

[fzhinkin]

@lppedd

so basically even setting

apiVersion = KotlinVersion.KOTLIN_1_9
languageVersion = KotlinVersion.KOTLIN_1_9

wouldn't help?

Neither of these options affects binary compatibility, so no, unfortunately, that won't help.

I'm just asking that because libraries suddenly requiring 2.0+ means whoever is using 1.9.x can't update anything. I've seen coroutines doing the same, serialization too. This seems a bit risky IMO.

For kotlinx-io, the new release will only switch the Kotlin version, so those who can't update the version in their projects can continue using 0.3.5 for a while.

Note that in terms of compatibility, there's nothing new here, and the same happened previously when libraries updated the Kotlin version from 1.8 to 1.9, for example.