Use Java 25 in native image system test

We need to use Java 25 when using buildpacks to create a native image
for a couple of reasons:

- Framework has raised its GraalVM baseline to 25
- Buildpacks have removed support for Java 24

See gh-45501

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

@@ -38,7 +38,7 @@ repositories {
 }
 
 tasks.named("bootBuildImage") {
-	environment["BP_JVM_VERSION"] = "24"
+	environment["BP_JVM_VERSION"] = "25"
 }
 
 dependencies {