Test against all Ruby versions (and a few more) listed in the README

Author: wconrad

.github/workflows/ruby.yml

@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: [ '3.2' ]
+        ruby-version: [ '2.7', '2.8', '2.9', '3.0', '3.1', '3.2', '3.3' ]
     steps:
     - uses: actions/checkout@v3
     - name: Set up Ruby