Add Rails 8.1 to CI matrix

Author: moritzhoeppner

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         ruby-version: ['3.1', '3.2', '3.3', '3.4']
-        rails-version: ['~> 7.2', '~> 8.0']
+        rails-version: ['~> 7.2', '~> 8.0', '~> 8.1']
         argon2-version: ['2.2', '2.3']
         orm:
           - adapter: active_record