Upgrade local dependencies to match what github pages uses

to match what is currently on https://pages.github.com/versions/

.github/workflows/docs.yml

@@ -15,6 +15,6 @@ jobs:
     - uses: actions/checkout@v2
     - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '2.6'
+        ruby-version: '2.7.1'
         bundler-cache: true
     - run: bundle exec jekyll build --trace