[BUG][Homebrew] Homebrew instructions for installing Java do not work

Created by: johnthagen

The instructions here: https://github.com/OpenAPITools/openapi-generator/blob/master/README.md#15---homebrew

To install OpenJDK (pre-requisites), please run

brew cask install adoptopenjdk12
export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk-12.0.2.jdk/Contents/Home/

Do not work when I try them (Latest Homebrew, macOS 10.15.5):

% brew cask install adoptopenjdk12
Updating Homebrew...
Error: Cask 'adoptopenjdk12' is unavailable: No Cask with this name exists.