Replace convention properties with extension blocks in build.gradle

The Gradle documentation says that these convention properties are
deprecated and superseded by the java and application extension blocks.
Using the convention properties doesn't generate warnings yet, but it
can't hurt to switch to the now recommended way.

build.gradle

@@ -46,8 +46,10 @@ dependencies {
 	implementation(group: "se.sawano.java", name: "alphanumeric-comparator", version: "1.4.1")
 }
 
-sourceCompatibility = "1.8"
-targetCompatibility = "1.8"
+java {
+	sourceCompatibility = JavaVersion.VERSION_1_8
+	targetCompatibility = JavaVersion.VERSION_1_8
+}
 
 tasks.withType(JavaCompile) {
 	options.encoding = SOURCE_ENCODING
@@ -71,13 +73,17 @@ processResources {
 	}
 }
 
-mainClassName = "de.prob2.jupyter.Main"
+application {
+	mainClass = "de.prob2.jupyter.Main"
+}
+// Workaround for https://github.com/johnrengelman/shadow/issues/609
+mainClassName = application.mainClass.get()
 
 final KERNEL_SPEC_OUTPUT_PATH = project.buildDir.toPath().resolve(Paths.get("kernelspec"))
 
 task createKernelSpec(type: JavaExec) {
 	dependsOn = [shadowJar]
-	main = project.mainClassName
+	mainClass = application.mainClass
 	args = ["createKernelSpec", KERNEL_SPEC_OUTPUT_PATH.toString()]
 	classpath(shadowJar.archiveFile)
 	if (project.hasProperty("probHome")) {