chore(internal): update actions/checkout version

Author: stainless-app[bot]

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up .NET
         uses: actions/setup-dotnet@v3
@@ -39,7 +39,7 @@ jobs:
     if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up .NET
         uses: actions/setup-dotnet@v3
@@ -55,7 +55,7 @@ jobs:
     runs-on: ${{ github.repository == 'stainless-sdks/orb-csharp' && 'depot-windows-2022' || 'windows-latest' }}
     if: github.event_name == 'push' || github.event.pull_request.head.repo.fork
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up .NET
         uses: actions/setup-dotnet@v5