Update java requirement in readme to Java 11

it is still compiled with binary compatibility of 8 but Java 11 is
required to do the build, otherwise there is this error:

```
[ERROR] Failed to execute goal
org.apache.maven.plugins:maven-compiler-plugin:3.10.1:compile(default-compile)
on project swagger-codegen-generators: Fatal error compiling: invalid
flag: --release
```

Signed-off-by: Aurélien Pupier <apupier@ibm.com>

Author: apupier

README.md

@@ -12,7 +12,7 @@ More details about these and more differences are referenced at [https://github.
 ### Prerequisites
 You need the following installed and available in your $PATH:
 
-* Java 8 (http://java.oracle.com)
+* Java 11+ (http://java.oracle.com)
 * Apache maven 3.0.4 or greater (http://maven.apache.org/)
 
 ## How to Contribute.