ci: drop 2.6

Signed-off-by: Kentaro Hayashi <[email protected]>

Author: kenhys

.github/workflows/linux-test.yaml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby-version: ['3.3', '3.2', '3.1', '3.0', '2.7', '2.6']
+        ruby-version: ['3.3', '3.2', '3.1', '3.0', '2.7']
         os: [ubuntu-latest]
         experimental: [false]
         include: