remove test for unsupported ruby 2.6, add for 3.1

Signed-off-by: Sebastian Gumprich <sebastian.gumprich@t-systems.com>

Author: Sebastian Gumprich

.github/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ['2.6', '2.7', '3.0']
+        ruby-version: ['2.7', '3.0', '3.1']
 
     steps:
     - uses: actions/checkout@v2