Update actions/setup-dotnet action to v6

Author: renovate[bot]

.github/workflows/main.yml

@@ -27,7 +27,7 @@ jobs:
 
       # Backend
       - name: Set up .NET SDK
-        uses: actions/setup-dotnet@v5
+        uses: actions/setup-dotnet@v6
         with:
           dotnet-version: '8.0.x'
       - name: NuGet Cache