Try to fix CI for Ruby 2.7

Author: sshaw

.github/workflows/main.yml

@@ -16,6 +16,7 @@ jobs:
     strategy:
       matrix:
         ruby: ['3.4', '3.3', '3.2', '3.1', '3.0', '2.7']
+        bundler: '2.4'
 
     steps:
       - uses: actions/checkout@v4