Pin the ant builds to a Java 17 release

The javac tasks set no release, so classes took whatever the build JDK emitted
and then outlived it in target/, where a later JDK too old to load them fails
on a class-version error that looks nothing like its cause. Pin all 37 tasks to
maven.compiler.release, as the jaxrs test resources already did; the samples
also default it to 17, since those are run standalone with plain ant.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>