Use Java 24 for native image system test

Upgrade to Java 24 for the native image system test to ensure that
the new rachability JSON is used.

See gh-45501

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/java/org/springframework/boot/image/paketo/PaketoBuilderTests.java

@@ -309,6 +309,9 @@ class PaketoBuilderTests {
 		this.gradleBuild
 			.expectDeprecationMessages("has been deprecated. This is scheduled to be removed in Gradle 9.0");
 		this.gradleBuild.expectDeprecationMessages("upgrading_version_8.html#deprecated_access_to_convention");
+		// these deprecations are from native image buildpacks
+		this.gradleBuild.expectDeprecationMessages("Using a deprecated option --report-unsupported-elements-at-runtime",
+				"The option is deprecated and will be removed in the future.");
 		writeMainClass();
 		String imageName = "paketo-integration/" + this.gradleBuild.getProjectDir().getName();
 		ImageReference imageReference = ImageReference.of(ImageName.of(imageName));

spring-boot-system-tests/spring-boot-image-tests/src/systemTest/resources/org/springframework/boot/image/paketo/PaketoBuilderTests-nativeApp.gradle

@@ -21,6 +21,10 @@ repositories {
 	spring.mavenRepositories()
 }
 
+tasks.named("bootBuildImage") {
+	environment["BP_JVM_VERSION"] = "24"
+}
+
 dependencies {
 	implementation("org.springframework.boot:spring-boot-starter-web:{bootVersion}")
 }