build: set JDK version 11 since most gradle plugins depend on 11 now

- fixes #4

build.gradle

@@ -135,8 +135,8 @@ configurations.configureEach {
 java {
     withSourcesJar()
     withJavadocJar()
-    sourceCompatibility(JavaVersion.VERSION_1_8)
-    targetCompatibility(JavaVersion.VERSION_1_8)
+    sourceCompatibility(JavaVersion.VERSION_11)
+    targetCompatibility(JavaVersion.VERSION_11)
 
     // needed for XML-Doclet to work (since Doclet changed again with Java 13)
     toolchain {