changing references java version from 21 to 17

the version 21 is still new and its not included in all the piscine.

subjects/java/piscine/Cat/README.md

@@ -71,6 +71,6 @@ $
 ### Notions
 
 [Command-Line Arguments](https://docs.oracle.com/javase/tutorial/essential/environment/cmdLineArgs.html)
-[File](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/nio/file/Files.html)
-[Standard Output](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/io/PrintStream.html)
-[IOException](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/io/IOException.html)
+[File](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/nio/file/Files.html)
+[Standard Output](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/PrintStream.html)
+[IOException](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/IOException.html)