Build on a JDK 21 system JVM via .build-jdk21

The artifact-diff-maven-plugin (deploy phase) requires a Java 21+ JVM to run
Maven. The buildpack otherwise selects JDK 17 as the system JVM for this Java 8
target, so the publish step failed to load the plugin. A .build-jdk21 marker
pins the Maven system JVM to 21. The compile target is unchanged
(project.build.targetJdk/releaseJdk = 8, --release 8), so the published
artifact stays Java 8 bytecode.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jasmith-hs