ci: drop ruby version from setup-ruby config

If we omit it, it will use .ruby-version by default, which is exactly
what we want! (I had simply cargo-culted the original config).

Author: peff

.github/workflows/ci.yml

@@ -10,7 +10,6 @@ jobs:
     - name: ruby setup
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7.3
         bundler-cache: true
 
     - name: rails setup