Support a Java 17 to Java 21 transition

[iay]

Java 21 is out now, and at some point we will want to deploy that in production. This will require us to have a testbed that includes validator containers based on Java 21.

I don't think this is pressing today, and it would complicate things a fair bit to add yet another axis (17/21 vs Xalan vs 0.9/0.10) but I want to have a placeholder for this work at some future point.

This will probably need support from md-validator. Note that both the md-validator branches today are based on Java 17. I hope we won't need to branch that any more; if we wait until 0.9 is done then it should be a question of parameterising the base Java container in the Dockerfile on the main branch, and generating both images in the same CI step.