Remove superfluous Gradle DSL classes for GraalVM

Gradle and Kotlin DSL did the same thing, this condenses it down to
only one class.

start-site/src/main/java/io/spring/start/site/extension/dependency/graalvm/GraalVmGradleBuildCustomizer.java

@@ -37,12 +37,6 @@ public final void customize(GradleBuild build) {
 		if (this.nbtVersion != null) {
 			build.plugins().add("org.graalvm.buildtools.native", (plugin) -> plugin.setVersion(this.nbtVersion));
 		}
-		// Spring Boot plugin
-		customizeSpringBootPlugin(build);
-	}
-
-	protected void customizeSpringBootPlugin(GradleBuild build) {
-
 	}
 
 }

start-site/src/main/java/io/spring/start/site/extension/dependency/graalvm/GraalVmProjectGenerationConfiguration.java

@@ -69,15 +69,9 @@ GraalVmMavenBuildCustomizer graalVmMavenBuildCustomizer() {
 	}
 
 	@Bean
-	@ConditionalOnBuildSystem(value = GradleBuildSystem.ID, dialect = GradleBuildSystem.DIALECT_GROOVY)
-	GraalVmGroovyDslGradleBuildCustomizer graalVmGroovyDslGradleBuildCustomizer() {
-		return new GraalVmGroovyDslGradleBuildCustomizer(this.nbtVersion.get());
-	}
-
-	@Bean
-	@ConditionalOnBuildSystem(value = GradleBuildSystem.ID, dialect = GradleBuildSystem.DIALECT_KOTLIN)
-	GraalVmKotlinDslGradleBuildCustomizer graalVmKotlinDslGradleBuildCustomizer() {
-		return new GraalVmKotlinDslGradleBuildCustomizer(this.nbtVersion.get());
+	@ConditionalOnBuildSystem(GradleBuildSystem.ID)
+	GraalVmGradleBuildCustomizer graalVmGradleBuildCustomizer() {
+		return new GraalVmGradleBuildCustomizer(this.nbtVersion.get());
 	}
 
 	@Bean

start-site/src/test/java/io/spring/start/site/extension/dependency/graalvm/GraalVmGradleBuildCustomizerTests.java

@@ -30,13 +30,11 @@
  *
  * @author Stephane Nicoll
  */
-abstract class GraalVmGradleBuildCustomizerTests {
-
-	protected abstract GraalVmGradleBuildCustomizer createCustomizer(String nbtVersion);
+class GraalVmGradleBuildCustomizerTests {
 
 	@Test
 	void gradleBuildConfigureNativeBuildToolsPlugin() {
-		GraalVmGradleBuildCustomizer customizer = createCustomizer("1.0.0");
+		GraalVmGradleBuildCustomizer customizer = new GraalVmGradleBuildCustomizer("1.0.0");
 		GradleBuild build = createBuild();
 		customizer.customize(build);
 		GradlePlugin nbtPlugin = build.plugins()