Add JDK21 info to doc #1017 (#1028)

Author: HapppyLiu

Diff for: project/project-info.conf

@@ -5,7 +5,7 @@ project-info {
   scaladoc: "https://pekko.apache.org/api/pekko/"${project-info.version}"/org/apache/pekko/"
   javadoc: "https://pekko.apache.org/japi/pekko/"${project-info.version}"/org/apache/pekko/"
   shared-info {
-    jdk-versions: ["OpenJDK 8", "OpenJDK 11", "OpenJDK 17"]
+    jdk-versions: ["OpenJDK 8", "OpenJDK 11", "OpenJDK 17", "OpenJDK 21"]
     snapshots: {
       url: "https://pekko.apache.org/docs/pekko/current/project/links.html#snapshots-repository"
       text: "Snapshots are available"