ci: add Ruby 4.0

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/workflows/linux.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4' ]
+        ruby: [ '2.4', '2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0' ]
         postgres: [ '9', '10', '11', '12', '13' ]
         os:
           - ubuntu-latest