Set auto_updates to true in Homebrew cask/monarch.json

Since monarch has a built-in auto updater, I think it would be appropriate to set:

`auto_updates = true` in

https://formulae.brew.sh/api/cask/monarch.json

That way Monarch does it’s own auto-update, without then being “updated” again through brew!

Also this would match other Brew casks with built-in auto updaters like Firefox.

This behavior would fit the brew philosophy:
> So we let software be. Anything installed with Homebrew Cask should behave the same as if it were installed manually. But since we also want to support software that doesn’t self-upgrade, we add auto_updates true to casks for software that does, which excludes them from brew upgrade.

See here:

https://docs.brew.sh/FAQ#why-arent-some-apps-included-during-brew-upgrade