Add banner for docker image Java 21 change (#8085)

* Add banner for docker image Java 21 change

* adjust formatting of image names

Author: kmartens27

content/_data/changelogs/weekly.yml

@@ -26617,6 +26617,11 @@
 
   - version: '2.509'
     date: 2025-05-06
+    banner: >
+      Starting with Jenkins 2.509, the default Docker images (<code>2.509</code>, <code>latest</code>, <code>alpine</code>, <code>slim</code>,
+      ...) use Java 21 unless specifically tagged with the <code>jdk17</code> string.
+      Refer to the <a href="https://github.com/jenkinsci/docker/releases/tag/2.509">Docker
+      2.509 changelog</a> or <a href="https://github.com/jenkinsci/docker/pull/2008">pull request for this update</a> for more details.
     changes:
       - type: major rfe
         category: major rfe