* Add Java 21 holders in JavadocTagConstants

Fixes the problem of no tags being considered valid for Java 21

* Run parser tests on Java 21

Signed-off-by: David Thompson <davthomp@redhat.com>

---------

Signed-off-by: David Thompson <davthomp@redhat.com>
Co-authored-by: David Thompson <davthomp@redhat.com>