Switch default JVM target to 1.8

#KT-29405 Fixed

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> {