Switch default JVM target to 1.8

#KT-29405 Fixed

compiler/testData/codegen/box/super/interfaceHashCode.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 interface I
 class C : I { fun foo() = super<I>.hashCode() }
 

compiler/testData/codegen/box/super/interfaceHashCode_1_6.kt

@@ -0,0 +1,11 @@
+// TARGET_BACKEND: JVM
+// IGNORE_BACKEND_FIR: JVM_IR
+// JVM_TARGET: 1.6
+
+interface I
+class C : I { fun foo() = super<I>.hashCode() }
+
+fun box(): String {
+    C().foo()
+    return "OK"
+}