Outdated Homebrew install protocol for cask

[crvernon]

Brief: The documented protocol for installing Java via cask from Homebrew on MacOSX is outdated.

My system specs: OS: macOS High Sierra, Version 10.13.6 Homebrew version: Homebrew 3.0.11

Summary: This references content within the "1. Introduction" section in the package managers table on page http://jgcri.github.io/gcam-doc/gcam-build.html

cask is no longer a brew command as of brew 2.6.0 Instead the user may simply use brew install or brew install --cask instead of brew cask install.

If brew cask install is used, the follow error will echo: Error: Unknown command: cask

See reference to this here: https://github.com/Homebrew/discussions/discussions/902

Recommended action:

1. Change command from brew cask install java to brew install java
2. Let users know that they need to run brew upgrade and use version 2.6.0 or greater