(maint) Update static code analysis Ruby version

The static code analysis GitHub Action runs on Ruby 2.6, which went
end-of-life on April 12, 2022. This commit updates the Ruby version
to Ruby 2.7, which is currently supported.

Author: mhashizume

.github/workflows/static_code_analysis.yaml

@@ -12,7 +12,7 @@ jobs:
     name: Run checks
 
     env:
-      ruby_version: 2.6
+      ruby_version: '2.7'
 
     runs-on: 'ubuntu-20.04'
     steps: