add java toolchain setup

mppJavaProject.gradle

@@ -28,3 +28,9 @@ kotlin {
         }
     }
 }
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(8)
+    }
+}

mppProjectWithSerialization.gradle

@@ -49,4 +49,10 @@ kotlin {
     }
 }
 
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(8)
+    }
+}
+
 apply from: "$defaultAndroidSettingsPresetPath"