Bump GH action

ref. https://github.com/actions/checkout/releases/tag/v6.0.1

Author: nil4

.github/workflows/ci.yml

@@ -54,7 +54,7 @@ jobs:
 
     steps:
     - name: checkout
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - uses: actions/setup-dotnet@v5
       with: