create-a-gpg-key: Remove outdated Homebrew install caveat
create-a-gpg-key: Remove outdated Homebrew install caveat

- This was added back when these docs were on GitHub Enterprise. I've
  not experienced these problems, and I would hope that they're fixed now.
- GPG Tools is excellent software, so recommend `brew cask install
  gpg-suite` for install and update consistency.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs