ci: don't try to update rubygems

because latest rubygems-update doesn't support Ruby 2.7

Author: flavorjones

.github/workflows/ci.yml

@@ -27,7 +27,6 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{matrix.ruby}}
-          rubygems: latest
           bundler: latest
           bundler-cache: true
       - name: Run tests