Test with Java 25 and Java 21 (jenkinsci#221)

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.

Testing done:

* Confirmed that automated tests pass with Java 25

Author: MarkEWaite

Jenkinsfile

@@ -6,6 +6,6 @@ buildPlugin(
   forkCount: '1C', // Run parallel tests on ci.jenkins.io for lower costs, faster feedback
   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],
 ])