Drop 2.3 from CI

Author: aycabta

.github/workflows/ubuntu.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x', '2.3.x' ]
+        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up Ruby