Support for both Java SE 17 and Java SE 21 in Concurrency 3.1

[njr-11]

A change in direction was recently announced that Jakarta EE 11 specifications must be built against Java SE 17 or lower rather than Java SE 21 (the previous plan). This impacts our Concurrency 3.1 release and will require some changes.

• API updates to support Java SE 17 #415 Update the Concurrency 3.1 API to remove usage Java SE 21. This will impact our API JavaDoc which currently links to directly into Java SE 21 API for virtual threads. We will need to get rid of those links to allow the project to be built against Java 17.
• TCK tests for virtual threads #425 Change how we write TCK tests to carefully avoid use of Java SE 21 or do so reflectively. This will impact tests scenarios like verifying that the managed executors can capture context from a virtual thread and propagate it to a managed executor thread, or that the context service can propagate context when running inline on a virtual thread, as well as other tests that need to identify whether a virtual thread is used in order to make assertions. These tests will need to become aware of the Java SE level and then conditionally skip (if less than 21) or invoke Java SE 21 methods reflectively or some other mechanism that allows Java SE 21 to be used. This will make the test code difficult to understand, but this seems to be the only option.
• Switch the project to compile against Java SE 17.