Switch default JVM target to 1.8

#KT-29405 Fixed

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinJvmOptions.kt

@@ -23,9 +23,9 @@ interface KotlinJvmOptions  : org.jetbrains.kotlin.gradle.dsl.KotlinCommonOption
      var jdkHome: kotlin.String?
 
     /**
-     * Target version of the generated JVM bytecode (1.6, 1.8, 9, 10, 11, 12, 13, 14 or 15), default is 1.6
+     * Target version of the generated JVM bytecode (1.6, 1.8, 9, 10, 11, 12, 13, 14 or 15), default is 1.8
      * Possible values: "1.6", "1.8", "9", "10", "11", "12", "13", "14", "15"
-     * Default value: "1.6"
+     * Default value: "1.8"
      */
      var jvmTarget: kotlin.String
 

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/dsl/KotlinJvmOptionsBase.kt

@@ -62,7 +62,7 @@ internal abstract class KotlinJvmOptionsBase : org.jetbrains.kotlin.gradle.dsl.K
 
     private var jvmTargetField: kotlin.String? = null
     override var jvmTarget: kotlin.String
-        get() = jvmTargetField ?: "1.6"
+        get() = jvmTargetField ?: "1.8"
         set(value) {
             jvmTargetField = value
         }
@@ -137,7 +137,7 @@ internal fun org.jetbrains.kotlin.cli.common.arguments.K2JVMCompilerArguments.fi
     includeRuntime = false
     javaParameters = false
     jdkHome = null
-    jvmTarget = "1.6"
+    jvmTarget = "1.8"
     moduleName = null
     noJdk = false
     noReflect = true