CI: Avoid 3.0 becoming "3" in YAML (#1001)

When stringifying 3.0, the CI's YAML engine would create the string "3" rather than "3.0". Turning this specific value into a String avoids this display difference.

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [3.0, 3.1, 3.2, 3.3, head, truffleruby-head]
+        ruby: ['3.0', 3.1, 3.2, 3.3, head, truffleruby-head]
     env:
       RAILS_ENV: test
     steps: