CI: Add Ruby 3.2 and drop 2.6

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Author: ashie

.github/workflows/linux.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '3.1', '3.0', '2.7', '2.6' ]
+        ruby: [ '3.2', '3.1', '3.0', '2.7' ]
         postgres: [ '14', '13', '12', '11', '10', '9' ]
         os:
           - ubuntu-latest