Fix KotlinCore build

KotlinCore/build.gradle.kts

@@ -18,6 +18,7 @@
  */
 
 plugins {
+    id("java")
     kotlin("jvm") version "2.0.0"
 
     id("com.github.johnrengelman.shadow")
@@ -26,6 +27,10 @@ plugins {
 group = "de.steamwar"
 version = ""
 
+tasks.compileJava {
+    options.encoding = "UTF-8"
+}
+
 tasks.build {
     finalizedBy(tasks.shadowJar)
 }
@@ -34,6 +39,11 @@ tasks.shadowJar {
     exclude("org/**")
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
+}
+
 sourceSets {
     main {
         java {