fix: add back deprecated mainClassName property

waiting for fix from PR johnrengelman/shadow#612 to be released in a new version of com.github.johnrengelman.shadow

(cherry picked from commit 4122bbf7fc8eaca494129dc65f3e380f331f6581)

build.gradle.kts

@@ -5,7 +5,7 @@ plugins {
     // Apply the application plugin to add support for building a CLI application
     // You can run your app via task "run": ./gradlew run
     application
-    
+
     /*
      * Adds tasks to export a runnable jar.
      * In order to create it, launch the "shadowJar" task.
@@ -60,10 +60,17 @@ tasks.withType<Test> {
 application {
     // Define the main class for the application
     mainClass.set("it.unibo.samplejavafx.App")
+
+    /*
+     * mainClassName was deprecated by Gradle, but it is still required by John Engelman's Shadow plugin.
+     * A pull request with a fix was already merged, but it hasn't been released yet;
+     * see https://github.com/johnrengelman/shadow/issues/609 and https://github.com/johnrengelman/shadow/pull/612
+     */
+    @Suppress("DEPRECATION")
+    mainClassName = mainClass.get()
 }
 
 java {
     sourceCompatibility = JavaVersion.VERSION_11
     targetCompatibility = JavaVersion.VERSION_11
 }
-