fix: replace deprecated API only if Gradle is newer or same to v6.1

Author: KengoTODA

src/main/java/org/javamodularity/moduleplugin/tasks/CompileModuleInfoTask.java

@@ -59,7 +59,15 @@ public void execute(Task task) {
             File moduleInfoDir = helper().getModuleInfoDir();
             jar.from(moduleInfoDir);
             jar.doFirst(task -> {
-                File classesDir = helper().mainSourceSet().getJava().getOutputDir();
+                File classesDir;
+                if (GradleVersion.current().compareTo(GradleVersion.version("6.1")) >= 0) {
+                    // SourceDirectorySet#getClassesDirectory() is supported from Gradle 6.1
+                    // https://docs.gradle.org/6.1/javadoc/org/gradle/api/file/SourceDirectorySet.html#getClassesDirectory--
+                    classesDir = helper().mainSourceSet().getJava().getClassesDirectory().get().getAsFile();
+                } else {
+                    classesDir = helper().mainSourceSet().getJava().getOutputDir();
+                }
+
                 File mainModuleInfoFile = new File(classesDir, "module-info.class");
                 File customModuleInfoFile = new File(moduleInfoDir, "module-info.class");
                 if(mainModuleInfoFile.isFile() && customModuleInfoFile.isFile()) {
@@ -82,7 +90,13 @@ private JavaCompile preconfigureCompileModuleInfoJava(JavaCompile compileJava) {
         compileModuleInfoJava.setSource(pathToModuleInfoJava());
         compileModuleInfoJava.getOptions().setSourcepath(project.files(pathToModuleInfoJava().getParent()));
 
-        compileModuleInfoJava.setDestinationDir(helper().getModuleInfoDir());
+        if (GradleVersion.current().compareTo(GradleVersion.version("6.1")) >= 0) {
+            // AbstractCompile#getDestinationDirectory() is supported from Gradle 6.1
+            // https://docs.gradle.org/6.1/javadoc/org/gradle/api/tasks/compile/AbstractCompile.html#getDestinationDirectory--
+            compileModuleInfoJava.getDestinationDirectory().set(helper().getModuleInfoDir());
+        } else {
+            compileModuleInfoJava.setDestinationDir(helper().getModuleInfoDir());
+        }
 
         // we need all the compiled classes before compiling module-info.java
         compileModuleInfoJava.dependsOn(compileJava);