Fixed Java version dependent on Flink.

As per this [site](https://nightlies.apache.org/flink/flink-docs-master/docs/deployment/java_compatibility/)., I clear Java 8 and add Java 17.
Adding Java 21 to beta based on contributor feedback.
The Maven version is modified to 3.9.0, the minimum version that rejects Java 8.

README.md

@@ -69,8 +69,8 @@ Prerequisites for building Flink:
 
 * Unix-like environment (we use Linux, Mac OS X, Cygwin, WSL)
 * Git
-* Maven (we require version 3.8.6)
-* Java 8 or 11 (Java 9 or 10 may work)
+* Maven (we require version 3.9.0)
+* Java 11, 17 or 21(Beta)
 
 ```
 git clone https://github.com/apache/flink.git