Set Java release version to 8 to fix Kotlin desugaring

Fixes https://github.com/pgpainless/pgpainless/issues/471

build.gradle

@@ -41,6 +41,10 @@ allprojects {
         options.release = 8
     }
 
+    tasks.withType(JavaCompile).configureEach {
+        options.release = 8
+    }
+
     // Only generate jar for submodules
     // without this we would generate an empty pgpainless.jar for the project root
     // https://stackoverflow.com/a/25445035