Add Ruby 3.3 to the test matrix

While here, quote versions, as `3.0` is considered as a floating point
value, and GitHub actions get confused when the fractional part of a
number is 0 and understand `3`, which in turn use the latest version of
Ruby 3.x (so, yes, this was already testing against Ruby 3.3 and in fact
add tests for 3.0, but don't tell anyone 😉).

.github/workflows/ci.yml

@@ -15,10 +15,11 @@ jobs:
     strategy:
       matrix:
         ruby-version:
-          - 2.7
-          - 3.0
-          - 3.1
-          - 3.2
+          - "2.7"
+          - "3.0"
+          - "3.1"
+          - "3.2"
+          - "3.3"
       fail-fast: false
     steps:
       - uses: actions/checkout@v2