Revert "Only test Ruby >=3"

This reverts commit 5c7f253.

Author: grega

.github/workflows/ruby.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['3.0', '3.1', '3.2', '3.3']
+        ruby-version: ['2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
 
     steps:
     - uses: actions/checkout@v2