gem: add Ruby 3.3 support, drop Ruby 2.7 due to EOL

.github/workflows/ci.yml

@@ -16,10 +16,10 @@ jobs:
     strategy:
       matrix:
         ruby-version:
-        - '2.7'
         - '3.0'
         - '3.1'
         - '3.2'
+        - '3.3'
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby ${{ matrix.ruby-version }}