Remove Rails 8/ruby 3.1 tests

Author: djmb

.github/workflows/main.yml

@@ -30,6 +30,9 @@ jobs:
         ruby-version: ["3.1", "3.2", "3.3"]
         gemfile: [ rails_7_2, rails_main ]
         database: [sqlite, postgres, mysql]
+        exclude:
+          - ruby-version: "3.1"
+            gemfile: rails_main
     services:
       redis:
         image: redis