Test with Ubuntu 18 only

Author: etagwerker

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04]
+        os: ['ubuntu-18.04']
         ruby: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2']
     steps:
     - uses: actions/checkout@v2