Update to Java 17

[dkfellows]

We should update the baseline language version to the current LTS version of Java. This would let us do quite a few things more efficiently (see #658), and also enables us to switch to using the current version of Spring, CXF, Jackson, Hibernate, and a bunch of other libraries (see #651).

We already support running with that version.

Key items to consider:

• Installation of Java 17 on relevant machines
• Picking correct installation (Adoptium or Zulu are currently recommended AIUI; Oracle is not because of license issues. We currently test against Zulu.)

[dkfellows]

Note that we currently support Java 17 — we include building with it and testing with it as part of our standard CI workflows, and have done since #497 — but don't currently require Java 17. This is about that step, i.e., about dropping support for Java 11 and 14.

[dkfellows]

The changes from 11 to 14 weren't large enough for us to justify updating for that. But #651 is a much stronger reason for us to care: JEE is the vendor-independent API specification for "enterprise Java" which includes just about everything to do with web services and validation.