Test with Java 25 and Java 21 (#1835)

Java 25 released September 16, 2025.  The Jenkins project wants to support
Java 25 soon.  Compile and test on ci.jenkins.io with Java 25 and Java 21.

Intentionally continues to generate Java 17 byte code as configured by
the plugin parent pom.

Does not compile or test with Java 17 on ci.jenkins.io any longer
because we have found no issues in the past that were specific to the
Java 17 compiler.  The plan is to drop support for Java 17 in the not
too distant future so that the Jenkins project is only supporting two
major Java versions at a time, Java 21 and Java 25.

Testing done:

* Confirmed that automated tests pass with Java 25