Documentation build now requires at least Java 17

Author: mrotteveel

README.md

@@ -28,7 +28,7 @@ How to build
 Run `./gradlew` or its full equivalent `./gradlew clean asciidoctor`. 
 Add `asciidoctorPdf` for the PDF version.
 
-Building the documentation requires Java 9 or higher, use of Java 17 or higher is advisable.
+Building the documentation requires Java 17 or higher.
 
 Supporting information
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