Quote Ruby versions in CI matrix

YAML parses unquoted `4.0` as a float and normalizes it to `4`, which
ruby/setup-ruby resolves to the latest 4.x release. Once 4.1 ships, the
"4.0" job would silently start testing 4.1 instead of 4.0. Quoting the
versions as strings pins them to the intended releases.

Author: walro

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ 3.2, 3.3, 3.4, 4.0, ruby-head ]
+        ruby: [ "3.2", "3.3", "3.4", "4.0", ruby-head ]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6