Test with Java 21

Java 21 released Sep 19, 2023. We'd like to announce full support for
Java 21 in early October and would like the most used plugins to be
compiling and testing with Java 21.

The acceptance test harness and plugin bill of materials tests are already
passing with Java 21. This is a further step to improve plugin readiness
for use with Java 21 and for development with Java 21.

Java 11 will be unsupported by Eclipse Temurin and some other Java
providers in October 2024. We'll need to move past Java 11 by that
time. It does not change the supported Java version or the byte code
that is being generated.

This intentionally does not include Java 11 in the test configuration
because Java 11 byte code is being generated by the Java 17 and Java 21
compilation and because Java 11 is being tested at least weekly by the
plugin bill of materials.

Let's save the expense of testing Java 11 on ci.jenkins.io and rely on
the existing tests (bom) of Java 11 and the Java 17 and Java 21 tests
that run with Java 11 byte code.

We could also consider switching this repository to test only Java 17 as
the preferred Java version for Jenkins controllers.  I'm open to either
Java 17 or Java 21 + Java 17.  The actual costs of running the tests in
this repository are small.

Jenkinsfile

@@ -1,5 +1,6 @@
 def configurations = [
-  [ platform: "linux", jdk: "11" ]
+  [ platform: "linux", jdk: "21" ],
+  [ platform: "linux", jdk: "17" ]
 ]
 
 buildPlugin(failFast: false, configurations: configurations)