Setup CI jobs on Ruby3 instead of Ruby 2.7

tested this in voxpupuli/puppet-collectd#966

Author: bastelfreak

moduleroot/.github/workflows/ci.yml.erb

@@ -18,7 +18,7 @@ jobs:
       - name: Setup ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '2.7'
+          ruby-version: '3.0'
           bundler-cache: true
       - name: Run static validations
         run: bundle exec rake validate lint check
@@ -87,7 +87,7 @@ jobs:
       - name: Setup ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '2.7'
+          ruby-version: '3.0'
           bundler-cache: true
       - name: Run tests
         run: bundle exec rake beaker