Set jdk_toolchain_version=17

[lunakoly]

Main PR to Kotlin: JetBrains/kotlin#7833
Alternative PR: #3250

^KT-88174

[sandwwraith]

I think it's better to set the whole toolchain. But we need to make sure that produced classfiles are still compatible with Java 8.

[lunakoly]

Check with Kotlin Master fails for an unrelated reason:

Execution failed for task ':kotlinx-serialization-core:compileKotlinJs'. org.gradle.workers.internal.DefaultWorkerExecutor$WorkExecutionException: A failure occurred while executing org.jetbrains.kotlin.compilerRunner.btapi.BuildToolsApiCompilationWork

Runtime library builds fail with (caused by the latest fixup, the previous build is green):

Execution failed for task ':guide:compileTestKotlin'. org.gradle.api.InvalidUserCodeException: ⛔ Inconsistent JVM Target Compatibility Between Java and Kotlin Tasks
Inconsistent JVM-target compatibility detected for tasks 'compileTestJava' (17) and 'compileTestKotlin' (1.8).
Solution: Consider using JVM Toolchain: https://kotl.in/gradle/jvm/toolchain
Learn more about JVM-target validation: https://kotl.in/gradle/jvm/target-validation