devel/jenkins-lts: update to 2.504.3 and use jdk21

Starting with Jenkins LTS 2.504.2 upstream uses JDK21 in published images.
Let's follow upstream and run on a modern JDK.
All FreeBSD architectures that have JDK17 also have JDK21.
NB: Jobs in Jenkins can/should use their own compiler version.

See also: https://www.jenkins.io/doc/book/platform-information/upgrade-java-to-21/index.html

PR: 287112
Approved by: lwhsu@ (maintainer)