Test with Java 25 and Java 21 (#265)

Java 25 released in September 2025 and the Jenkins project wants to
support it soon after release.  Don't test with Java 17, since we've not
found any issues that were specific to the Java version used for the test.

Testing done:

Confirmed that tests pass with Java 25

Author: MarkEWaite

Jenkinsfile

@@ -6,6 +6,6 @@ buildPlugin(
   forkCount: '1C', // Run a JVM per core in tests
   useContainerAgent: true, // Set to `false` if you need to use Docker for containerized tests
   configurations: [
-    [platform: 'linux', jdk: 21],
-    [platform: 'windows', jdk: 17],
+    [platform: 'linux', jdk: 25],
+    [platform: 'windows', jdk: 21],
 ])