Use brew version to check latest version

[jribbink]

We should use homebrew instead since version.txt is being deprecated on the Flow CLI repo (see onflow/flow-cli#1545). Either use the formula on github or homebrew API (must check what availability is like? rate limits?).

There is an implicit guarantee that the shell script install method will at least support the same version that Homebrew does, since Homebrew is the limiting factor for version updates.