README: Update required Java version (antlr#4129)

The badge in the README suggests that ANTLR requires Java 7 or higher,
whereas since ANTLR v4.10 Java 11 or higher is required.

Signed-off-by: Robert Adam <[email protected]>

Author: Krzmbrzl

README.md

@@ -1,6 +1,6 @@
 # ANTLR v4
 
-[![Java 7+](https://img.shields.io/badge/java-7+-4c7e9f.svg)](http://java.oracle.com)
+[![Java 11+](https://img.shields.io/badge/java-11+-4c7e9f.svg)](http://java.oracle.com)
 [![License](https://img.shields.io/badge/license-BSD-blue.svg)](https://raw.githubusercontent.com/antlr/antlr4/master/LICENSE.txt)