Simplify the "version already exists" check by using RubyGems v2 API (#…

…11712)

The v2 API has endpoints specific to each version: https://guides.rubygems.org/rubygems-org-api-v2/

Benefits:

1. Less data gets returned over the wire
    Compare:
       * https://rubygems.org/api/v1/versions/dependabot-omnibus.json
        * https://rubygems.org/api/v2/rubygems/dependabot-omnibus/versions/0.299.1.json
2. Less brittle since we no longer parse the `response.body`
3. Faster since we only check `response.code`

I verified that sending a non-existent version returns a `404`:
https://rubygems.org/api/v2/rubygems/dependabot-omnibus/versions/0.7.1.json

Rakefile

@@ -127,12 +127,8 @@ def guard_tag_match
 end
 
 def rubygems_release_exists?(name, version)
-  uri = URI.parse("https://rubygems.org/api/v1/versions/#{name}.json")
+  uri = URI.parse("https://rubygems.org/api/v2/rubygems/#{name}/versions/#{version}.json")
   response = Net::HTTP.get_response(uri)
-  return false if response.code != "200"
-
-  body = JSON.parse(response.body)
-  existing_versions = body.map { |b| b["number"] }
-  existing_versions.include?(version)
+  response.code == "200"
 end
 # rubocop:enable Metrics/BlockLength