Update min kotlin version to 1.8 (#7155)

Author: jack-berg

VERSIONING.md

@@ -73,7 +73,7 @@ respect to semantic versioning.
 |----------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | Java     | 8+                                                                                                                                                                                                                               | All artifacts, unless otherwise noted                   | Changing requires major version bump.                                                                                                                                              |
 | Android  | 23+ (NOTE: [desugaring](https://developer.android.com/studio/write/java8-support#library-desugaring) is required. We stay up to date with the latest version of [desugar_jdk_libs](https://github.com/google/desugar_jdk_libs).) | Artifacts using `otel.animalsniffer-conventions` plugin | Kept in sync with minimum requirements for [Google Play services](https://developers.google.com/android/guides/setup). Subject to change in minor version.                         |
-| Kotlin   | 1.6+                                                                                                                                                                                                                             | Only applies to `opentelemetry-extension-kotlin`        | Kept in sync with [minimum non-deprecated](https://kotlinlang.org/docs/gradle-compiler-options.html#attributes-common-to-jvm-and-js) version. Subject to change in minor versions. |
+| Kotlin   | 1.8+                                                                                                                                                                                                                             | Only applies to `opentelemetry-extension-kotlin`        | Kept in sync with [minimum non-deprecated](https://kotlinlang.org/docs/gradle-compiler-options.html#attributes-common-to-jvm-and-js) version. Subject to change in minor versions. |
 
 ## API vs SDK
 

docs/apidiffs/current_vs_latest/opentelemetry-extension-kotlin.txt

@@ -1,2 +1,9 @@
 Comparing source compatibility of opentelemetry-extension-kotlin-1.50.0-SNAPSHOT.jar against opentelemetry-extension-kotlin-1.49.0.jar
-No changes.
+===  UNCHANGED CLASS: PUBLIC FINAL io.opentelemetry.extension.kotlin.ContextExtensionsKt  (not serializable)
+	===  CLASS FILE FORMAT VERSION: 52.0 <- 52.0
+	***  MODIFIED ANNOTATION: kotlin.Metadata
+		===  UNCHANGED ELEMENT: xi=48
+		***  MODIFIED ELEMENT: mv=2,1,0 (<- 1,6,0)
+		===  UNCHANGED ELEMENT: k=2
+		===  UNCHANGED ELEMENT: d1=��&#xA;�&#xA;���&#xA;���&#xA;���&#xA;����&#xA;����0�*�0��&#xA;����0�*�0��&#xA;����0�*�0�¨��
+		===  UNCHANGED ELEMENT: d2=asContextElement,Lkotlin/coroutines/CoroutineContext;,Lio/opentelemetry/context/Context;,Lio/opentelemetry/context/ImplicitContextKeyed;,getOpenTelemetryContext,opentelemetry-extension-kotlin

extensions/kotlin/build.gradle.kts

@@ -1,4 +1,6 @@
-import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+import org.jetbrains.kotlin.gradle.tasks.KotlinJvmCompile
 
 plugins {
   id("otel.java-conventions")
@@ -45,10 +47,10 @@ testing {
 }
 
 tasks {
-  withType(KotlinCompile::class) {
-    kotlinOptions {
-      jvmTarget = "1.8"
-      languageVersion = "1.6"
+  withType(KotlinJvmCompile::class) {
+    compilerOptions {
+      jvmTarget.set(JvmTarget.JVM_1_8)
+      apiVersion.set(KotlinVersion.KOTLIN_1_8)
     }
   }