Drop ruby 2.6 from CI.

Author: vcsjones

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.6', '2.7', '3.0', '3.1', '3.2', '3.4', '4.0.0-preview2' ]
+        ruby: [ '2.7', '3.0', '3.1', '3.2', '3.4', '4.0.0-preview2' ]
 
     steps:
     - uses: actions/checkout@v4