Switch default JVM target to 1.8

#KT-29405 Fixed

idea/idea-gradle/tests/org/jetbrains/kotlin/idea/codeInsight/gradle/GradleFacetImportTest.kt

@@ -104,7 +104,7 @@ class GradleFacetImportTest : GradleImportingTestCase() {
             Assert.assertFalse(compilerArguments!!.autoAdvanceLanguageVersion)
             Assert.assertFalse(compilerArguments!!.autoAdvanceApiVersion)
             Assert.assertEquals(JvmPlatforms.jvm16, targetPlatform)
-            Assert.assertEquals("1.6", (compilerArguments as K2JVMCompilerArguments).jvmTarget)
+            Assert.assertEquals("1.8", (compilerArguments as K2JVMCompilerArguments).jvmTarget)
             Assert.assertEquals(
                 "-Xallow-no-source-files -Xdump-declarations-to=tmpTest",
                 compilerSettings!!.additionalArguments
@@ -158,7 +158,7 @@ class GradleFacetImportTest : GradleImportingTestCase() {
             Assert.assertEquals("1.3", languageLevel!!.versionString)
             Assert.assertEquals("1.0", apiLevel!!.versionString)
             Assert.assertEquals(JvmPlatforms.jvm16, targetPlatform)
-            Assert.assertEquals("1.6", (compilerArguments as K2JVMCompilerArguments).jvmTarget)
+            Assert.assertEquals("1.8", (compilerArguments as K2JVMCompilerArguments).jvmTarget)
             Assert.assertEquals(
                 "-Xallow-no-source-files -Xdump-declarations-to=tmpTest",
                 compilerSettings!!.additionalArguments

idea/idea-jps-common/src/org/jetbrains/kotlin/platform/impl/JvmIdePlatformKind.kt

@@ -38,7 +38,7 @@ object JvmIdePlatformKind : IdePlatformKind<JvmIdePlatformKind>() {
         message = "IdePlatform is deprecated and will be removed soon, please, migrate to org.jetbrains.kotlin.platform.TargetPlatform",
         level = DeprecationLevel.ERROR
     )
-    override fun getDefaultPlatform(): Platform = Platform(JvmTarget.JVM_1_6)
+    override fun getDefaultPlatform(): Platform = Platform(JvmTarget.DEFAULT)
 
     override fun createArguments(): CommonCompilerArguments {
         return K2JVMCompilerArguments()

idea/testData/checker/JvmStaticUsagesRuntime.kt

@@ -31,7 +31,7 @@ class A {
 <error descr="[WRONG_ANNOTATION_TARGET] This annotation is not applicable to target 'interface'">@JvmStatic</error>
 interface B {
     companion object {
-        <error descr="[JVM_STATIC_IN_INTERFACE_1_6] '@JvmStatic' annotation in interface supported only with JVM target 1.8 and above. Recompile with '-jvm-target 1.8'\"">@JvmStatic fun a1()</error> {
+        @JvmStatic fun a1() {
 
         }
     }

idea/testData/codeInsight/overrideImplement/jdk8/overrideCollectionStream.kt.after

@@ -1,4 +1,3 @@
-// ERROR: Super calls to Java default methods are prohibited in JVM target 1.6. Recompile with '-jvm-target 1.8'
 import java.util.stream.Stream
 
 abstract class A : List<String> {