Add HOMEBREW_UPGRADE_GREEDY="auto-updates" and "latest" about brew

[reitermarkus]

Given we already have HOMEBREW_UPGRADE_GREEDY, I think it would be better to support HOMEBREW_UPGRADE_GREEDY=auto-updates or HOMEBREW_UPGRADE_GREEDY_AUTO_UPDATES as you already mentioned.

Also, I think we should deprecate HOMEBREW_CASK_OPTS at some point and split it into separate variables for each supported option.

[jck112]

I'm happy to update my pull request with any preferred approach. Do you have a preference between adding HOMEBREW_UPGRADE_GREEDY_AUTO_UPDATES or modifying HOMEBREW_UPGRADE_GREEDY? If the latter, would it be backwards compatible to convert the value to a string and have any value != "auto-updates" imply the --greedy option?

[reitermarkus]

Let's modify HOMEBREW_UPGRADE_GREEDY to allow auto-updates. This way also only one of the options can be set at a time.

[jck112]

Updated #16736 to modify HOMEBREW_UPGRADE_GREEDY as requested.

[MikeMcQuaid]

Closing this out as we don't need an issue and PR from same author at same time and to keep conversation in one place. Thanks for the PR @jck112!