Remove Ruby 2.3 from CI, add 3.1

protobuf gem fails to build under 2.3

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
-        ruby: [2.3, 2.4, 2.5, 2.6, 2.7, '3.0']
+        ruby: [2.4, 2.5, 2.6, 2.7, '3.0', '3.1']
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2