Document that building Closure Compiler from source requires >= Java 21

Fixes #4209

PiperOrigin-RevId: 713831322

Author: lauraharker

README.md

@@ -365,7 +365,7 @@ To build the compiler yourself, you will need the following:
 
 Prerequisite                                                               | Description
 -------------------------------------------------------------------------- | -----------
-[Java 11 or later](https://java.com)                                       | Used to compile the compiler's source code.
+[Java 21 or later](https://java.com)                                       | Used to compile the compiler's source code.
 [NodeJS](https://nodejs.org)                                               | Used to generate resources used by Java compilation
 [Git](https://git-scm.com/)                                                | Used by Bazel to download dependencies.
 [Bazelisk](https://bazel.build/install/bazelisk) | Used to build the various compiler targets.