Update oga requirement from ~> 2.0 to ~> 3.0

Updates the requirements on [oga](https://gitlab.com/yorickpeterse/oga) to permit the latest version.
- [Release notes](https://gitlab.com/yorickpeterse/oga/tags)
- [Changelog](https://gitlab.com/yorickpeterse/oga/blob/master/CHANGELOG.md)
- [Commits](https://gitlab.com/yorickpeterse/oga/commits/master)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Gemfile

@@ -5,7 +5,7 @@ source "https://rubygems.org"
 gemspec
 
 gem "nokogiri", "~> 1.8"
-gem "oga", "~> 2.0"
+gem "oga", "~> 3.0"
 gem "rubocop", "~> 0.74"
 
 group :test do