Update minimum compilation to Java 21 (#743)

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Tim Jacomb <21194782+timja@users.noreply.github.com>

Author: dependabot[bot]

Jenkinsfile

@@ -4,6 +4,6 @@
  * allowing one to test against multiple Jenkins versions.
  */
 buildPlugin(useContainerAgent: true, configurations: [
-  [platform: 'linux', jdk: 21],
-  [platform: 'windows', jdk: 17],
+  [platform: 'linux', jdk: 25],
+  [platform: 'windows', jdk: 21],
 ])

pom.xml

@@ -4,7 +4,7 @@
   <parent>
     <groupId>org.jenkins-ci</groupId>
     <artifactId>jenkins</artifactId>
-    <version>1.142</version>
+    <version>2.1326.v00b_e26755312</version>
     <relativePath />
   </parent>