[gem] clarify the platform qualifier for rubygems

[prabhu]

The guidance says that the platform defaults to ruby.

https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst#gem

However, Gemfile.lock files specify a list of platforms and make a subtle distinction between platforms and Ruby VM. RubyGems API, however, works around the problem by supporting ruby_version in addition to the platform.

E.g.:

PLATFORMS
  aarch64-linux
  arm64-darwin
  ruby
  x86_64-darwin
  x86_64-linux

Doc: https://bundler.io/v2.5/man/bundle-platform.1.html Rubygems API: https://guides.rubygems.org/rubygems-org-api-v2/