Create a RuboCop to forbid dynamic caveats

[MikeMcQuaid]

Verification

• This issue's title and/or description do not reference a single formula e.g. brew install wget. If they do, open an issue at https://github.com/Homebrew/homebrew-core/issues/new/choose instead.

Provide a detailed description of the proposed feature

There should be a new Rubocop for brew style to be able to forbid dynamic caveats.

What is the motivation for the feature?

Homebrew/homebrew-core#208117 is an example where these caveats won't be displayed at all when using the JSON API (i.e. the vast majority of our users).

This would also make caveats easier/better to more usefully display on formulae.brew.sh where they are currently too dynamic.

How will the feature be relevant to at least 90% of Homebrew users?

Let's them actually see correct caveats.

What alternatives to the feature have been considered?

Not caring about this.