CI: Add Ruby 3.3 to test matrix

Ruby 3.3.0 has been released on 25 Dec 2023.

-- https://www.ruby-lang.org/en/downloads/releases/

Author: amotl

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, macos-latest ] # , windows-latest ]
-        ruby-version: [ "2.4", "2.5", "2.6", "2.7", "3.0", "3.1", "3.2" ]
+        ruby-version: [ "2.4", "2.5", "2.6", "2.7", "3.0", "3.1", "3.2", "3.3" ]
 
     name: Ruby ${{ matrix.ruby-version }} on OS ${{ matrix.os }}
     steps: