[CI] Rails 8.1 does not support Ruby 3.1

Author: amatsuda

.github/workflows/main.yml

@@ -98,6 +98,13 @@ jobs:
             rails_version: '4.2'
             bundler_version: '1'
 
+        exclude:
+          - ruby_version: '3.1'
+            rails_version: edge
+          - ruby_version: '3.1'
+            rails_version: edge
+            api: '1'
+
     env:
       RAILS_VERSION: ${{ matrix.rails_version }}
       API: ${{ matrix.api }}