Use ruby/setup-ruby

It supports better ruby versions support.
- https://github.com/ruby/setup-ruby

This PR is similar to lostisland/faraday#1187.

.github/workflows/ci.yml

@@ -18,9 +18,9 @@ jobs:
       - uses: actions/checkout@v1
 
       - name: Set up Ruby 2.7
-        uses: actions/setup-ruby@v1
+        uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.7.x
+          ruby-version: 2.7
 
       - name: Rubocop
         run: |
@@ -38,7 +38,7 @@ jobs:
       - uses: actions/checkout@v1
 
       - name: Set up Ruby
-        uses: actions/setup-ruby@v1
+        uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}