chore: update CI to Rails 8.1.1 and checkout@v5

Author: seuros

.github/workflows/ci.yml

@@ -47,11 +47,11 @@ jobs:
           - '3.4'
         rails:
           - '8.0'
-          - '8.1.0.rc1'
+          - '8.1.1'
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
 
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1