Deprecate `gpg` stanza.

Library/Homebrew/cask/lib/hbc/cask.rb

Library/Homebrew/cask/lib/hbc/dsl.rb

Library/Homebrew/cask/lib/hbc/dsl/gpg.rb

Library/Homebrew/cask/lib/hbc/installer.rb

Library/Homebrew/cask/lib/hbc/verify.rb

Library/Homebrew/cask/lib/hbc/verify/checksum.rb

Library/Homebrew/cask/lib/hbc/verify/gpg.rb

Library/Homebrew/compat/hbc/dsl.rb

Library/Homebrew/test/cask/cli/install_spec.rb

Library/Homebrew/test/cask/cli/internal_stanza_spec.rb

Library/Homebrew/test/cask/cli/reinstall_spec.rb

Library/Homebrew/test/cask/dsl_spec.rb

Library/Homebrew/test/cask/installer_spec.rb

...xtures/cask/Casks/invalid/invalid-gpg-conflicting-keys.rb

...support/fixtures/cask/Casks/invalid/invalid-gpg-key-id.rb

...upport/fixtures/cask/Casks/invalid/invalid-gpg-key-url.rb

...rt/fixtures/cask/Casks/invalid/invalid-gpg-missing-key.rb

...xtures/cask/Casks/invalid/invalid-gpg-multiple-stanzas.rb

...port/fixtures/cask/Casks/invalid/invalid-gpg-parameter.rb

.../fixtures/cask/Casks/invalid/invalid-gpg-signature-url.rb