Enforce JDK 21+ as minimum build JDK for multi-release JAR support

[Croway]

Summary

• Add requireJavaVersion [21,) to the existing enforce-java-version execution in maven-enforcer-plugin
• Change Jenkinsfile.deploy default JDK from jdk_17_latest to jdk_21_latest
• Update minimalJavaBuildVersion from ${jdk.version} (17) to 21

Motivation

camel-util uses multi-release JARs with a java-21-sources profile activated by <jdk>[21,)</jdk>. If Camel is built with JDK 17, the virtual thread classes under META-INF/versions/21/ are silently omitted — users running on JDK 21 would get JARs without virtual thread support.

Jenkinsfile.deploy defaults to jdk_17_latest, meaning daily SNAPSHOT deployments were built without virtual thread support.

This does not change the runtime requirement — Camel JARs remain Java 17 compatible. It only ensures the build JDK is high enough to compile the multi-release JAR layers.

The enforcer runs only when the full profile is active (not with -Dquickly), so fast local builds are unaffected. The Jenkinsfile.deploy fix ensures SNAPSHOT deployments use JDK 21.

Backport of #22608

Discussion thread: dev@camel.apache.org "[DISCUSS] Enforce JDK 21+ for building Camel"