Warn users in March 2029 of Java 25 end of support September 2029 (#25925)

Warn users of Java 25 end of support September 2029

Beginning in March 2029, users will be warned that Java 25 will be
reaching its end of support in the Jenkins project.

Amends pull request:

* https://github.com/jenkinsci/jenkins/pull/11148

The 2+2+2 Java support plan recommends that we drop support for a Java
line after two years so that we're only supporting two Java versions at
a time.  Refer to the blog post:

* https://www.jenkins.io/blog/2023/11/06/introducing-2-2-2-java-support-plan/

Also refer to the Jenkins Enhancement Proposal pull request (incomplete):

* https://github.com/jenkinsci/jep/pull/400

Testing done:

* None.  Relying on the existing behavior that I've seen from the Java
  17 administrative monitor.  It is now shown to users to warn them that
  we plan to drop Java 17 support by the end of March 2026

core/src/main/java/jenkins/monitor/JavaVersionRecommendationAdminMonitor.java

@@ -80,6 +80,7 @@ public class JavaVersionRecommendationAdminMonitor extends AdministrativeMonitor
         NavigableMap<Integer, LocalDate> supportedVersions = new TreeMap<>();
         supportedVersions.put(17, LocalDate.of(2026, 3, 31)); // Temurin: 2027-10-31
         supportedVersions.put(21, LocalDate.of(2027, 9, 30)); // Temurin: 2029-09-30
+        supportedVersions.put(25, LocalDate.of(2029, 9, 30)); // Temurin: 2031-09-30
         SUPPORTED_JAVA_VERSIONS = Collections.unmodifiableNavigableMap(supportedVersions);
     }