Update embedded Kotlin and Kotlin language level to 1.8

[ljacomet]

Gradle 7.x uses Kotlin 1.4 to 1.6 depending on the exact version. However the Kotlin language level produced by script compilation remains at 1.4.

Gradle 8.0 should upgrade embedded Kotlin to 1.8 and most likely start targeting 1.8 language level.

Related issues:

• https://github.com/gradle/gradle/pull/23214
• https://github.com/gradle/gradle/pull/23749