dep: update loofah and nokogiri to versions fully supporting HTML5

[flavorjones]

THIS IS VARIATION 2 - see variation 1 at #164

Require versions of Nokogiri and Loofah that support HTML5:

• loofah v2.21.x introduced Rails::HTML5::Sanitizer
• nokogiri v1.14.x is needed for loofah to subclass Nokogiri::HTML5::{Document,DocumentFragment}

Update required_ruby_version to ">= 2.7.0" to match Nokogiri's constraint for v1.14.x.

Update CI to only test supported Ruby versions.

The alternative (#164) is to continue to support older versions of Nokogiri (for Ruby 2.5) and Loofah.