CI: Add Ruby 3.0 to build matrix

Author: olleolleolle

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [2.5, 2.6, 2.7, head, truffleruby-head]
+        ruby: [2.5, 2.6, 2.7, "3.0", head, truffleruby-head]
     env:
       RAILS_ENV: test
     steps: