[ruby/date] Provide a 'Changelog' link on rubygems.org/gems/date

By providing a 'changelog_uri' in the metadata of the gemspec
a 'Changelog' link will be shown on https://rubygems.org/gems/date
which makes it quick and easy for someone to check on the changes
introduced with a new version.

Details of this functionality can be found on https://guides.rubygems.org/specification-reference/

ruby/date@2dea4c011d

ext/date/date.gemspec

@@ -31,4 +31,6 @@ Gem::Specification.new do |s|
   s.email = [nil]
   s.homepage = "https://github.com/ruby/date"
   s.licenses = ["Ruby", "BSD-2-Clause"]
+
+  s.metadata["changelog_uri"] = s.homepage + "/releases"
 end