[Build] Move sourceCompatibility option to the root build file

build.gradle

@@ -3,9 +3,14 @@ import tool.UpVersion
 allprojects {
     group 'imgui-java'
     version property('version')
+
     repositories {
         mavenCentral()
     }
+
+    tasks.withType(JavaCompile) {
+        sourceCompatibility = JavaVersion.VERSION_1_8
+    }
 }
 
 tasks.register('upVersion', UpVersion)