MINOR: Replace Java 14 with Java 15 in the README (#9298)

We have been testing with the Java 15 release candidate for a few weeks and
it has now been declared final.

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>, David Jacot <djacot@confluent.io>, Lee Dongjin <dongjin@apache.org>

README.md

@@ -4,7 +4,7 @@ See our [web site](https://kafka.apache.org) for details on the project.
 
 You need to have [Java](http://www.oracle.com/technetwork/java/javase/downloads/index.html) installed.
 
-We build and test Apache Kafka with Java 8, 11 and 14. We set the `release` parameter in javac and scalac
+We build and test Apache Kafka with Java 8, 11 and 15. We set the `release` parameter in javac and scalac
 to `8` to ensure the generated binaries are compatible with Java 8 or higher (independently of the Java version
 used for compilation).