Add Rails 8.1 to workflows

Author: jas14

.github/workflows/super_diff.yml

@@ -61,12 +61,15 @@ jobs:
           - rails_7_1
           - rails_7_2
           - rails_8_0
+          - rails_8_1
           - no_rails
         rspec_appraisal:
           - rspec_lt_3_10
           - rspec_gte_3_10
           - rspec_gte_3_13
         exclude:
+        - ruby: "3.1"
+          rails_appraisal: "rails_8_1"
         - ruby: "3.1"
           rails_appraisal: "rails_8_0"
         - ruby: "3.4"