Upgrade Kotlin from 1.6.21 to 2.x

[lucassaldanha]

Context

During a dependency update pass (see PR #424 and related work), several dependencies could not be upgraded because they require Kotlin 2.x:

Dependency	Blocked version	Reason
io.mockk:mockk	1.14.x	Transitively requires kotlin-stdlib:2.1.20
com.github.ben-manes.versions Gradle plugin	0.54.0	Requires Kotlin 2.x compiler
io.gitlab.arturbosch.detekt	1.23.x	Uses Kotlin 1.9 compiler API
org.jetbrains.kotlinx:kotlinx-coroutines-core	1.10.x	Requires Kotlin 1.8+

What needs to happen

• Upgrade org.jetbrains.kotlin.jvm plugin from 1.6.21 to 2.x (latest stable: 2.1.20)
• Update languageVersion in KotlinCompile options (currently "1.6")
• Upgrade kotlinx-coroutines-core to 1.10.x
• Verify allWarningsAsErrors = true still passes (new compiler warnings are expected)
• Re-test all modules after the upgrade
• Unblock the deferred dependency upgrades listed above

Notes

• The Kotlin 2.x migration guide should be consulted: https://kotlinlang.org/docs/k2-compiler-migration-guide.html
• Android example module (examples/android-chatter) may need separate attention given its AGP version constraint