Update Readme and Screenshot

Change-Id: I683cafe38e0c2881d5024ff79821e1ac707714c9
diff --git a/README.md b/README.md
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 ## Prerequisites
 
-Java 17 (OpenJDK or Oracle JDK),
+Java 21 (OpenJDK or Oracle JDK),
 [Git](http://git-scm.com/),
 [Maven 3](https://maven.apache.org/).
 Further dependencies are resolved using Maven.
diff --git a/misc/kalamar-export-screenshot.png b/misc/kalamar-export-screenshot.png
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