(maint) Update check Ruby from 2.5 to 2.7

Author: mhashizume

.github/workflows/checks.yaml

@@ -16,9 +16,9 @@ jobs:
     strategy:
       matrix:
         cfg:
-          - {check: rubocop, os: ubuntu-latest, ruby: 2.5}
-          - {check: commits, os: ubuntu-latest, ruby: 2.5}
-          - {check: warnings, os: ubuntu-latest, ruby: 2.5}
+          - {check: rubocop, os: ubuntu-latest, ruby: '2.7'}
+          - {check: commits, os: ubuntu-latest, ruby: '2.7'}
+          - {check: warnings, os: ubuntu-latest, ruby: '2.7'}
 
     runs-on: ${{ matrix.cfg.os }}
     steps: