Upgrade to Kotlin 1.8

This commit also disables the JVM target validation
introduced in Kotlin 1.8 since Kotlin is not yet
compatible with Java 21.

Closes gh-29754

gradle.properties

@@ -4,6 +4,7 @@ org.gradle.caching=true
 org.gradle.jvmargs=-Xmx2048m
 org.gradle.parallel=true
 
-kotlinVersion=1.7.21
+kotlinVersion=1.8.21
 
 kotlin.stdlib.default.dependency=false
+kotlin.jvm.target.validation.mode=ignore