Upgrade to Gradle 9 (#251)

This upgrades Gradle to 9.1.0 to be consistent with
exercism/java-test-runner#171.

This has also required the following changes:
- Updating the Shadow plugin to a compatible version (see
  https://github.com/GradleUp/shadow).
- Adding JUnit launcher as test runtime dependency, as the automatic
  loading of test framework implementation was removed
  (see https://docs.gradle.org/8.7/userguide/upgrading_version_8.html#test_framework_implementation_dependencies).
- Replacing the mainClassName property. This was deprecated back
  in Gradle 8 (see https://docs.gradle.org/current/userguide/upgrading_version_7.html#javaapplication_api_cleanup).

Author: kahgoh

Dockerfile

@@ -1,4 +1,4 @@
-FROM gradle:8.7-jdk21 AS build
+FROM gradle:9.1.0-jdk25 AS build
 
 WORKDIR /app
 COPY . /app

build.gradle

@@ -2,12 +2,15 @@ plugins {
     id "java"
     id "application"
     id "jacoco"
-    id "com.github.johnrengelman.shadow" version "8.1.1"
+    id 'com.gradleup.shadow' version '9.2.2'
 }
 
 group = "org.exercism"
 version = "1.0-SNAPSHOT"
-mainClassName = "representer.RepresenterCli"
+
+application {
+    mainClass = "representer.RepresenterCli"
+}
 
 repositories {
     mavenCentral()
@@ -25,6 +28,8 @@ dependencies {
     testImplementation "org.junit.jupiter:junit-jupiter"
     testImplementation "org.assertj:assertj-core:3.25.3"
     testImplementation "com.approvaltests:approvaltests:25.4.3"
+
+    testRuntimeOnly "org.junit.platform:junit-platform-launcher"
 }
 
 jar {

gradle/wrapper/gradle-wrapper.jar

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.7-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.1.0-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME