fix: surround ruby versions with quotes to fix the broken CI matrix

Version 3.0 was treated as "3", leading to testing the latest 3.X. I also added the intermediate versions for thorough coverage

.github/workflows/main.yml

@@ -31,7 +31,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [2.3, 2.7, 3.0]
+        ruby: ['2.3', '2.7', '3.0', '3.1', '3.2', '3.3']
     name: install dependencies and run tests
     steps:
       - name: install image processing dependencies