Fix CI: update .NET SDK version to 9.0.x and actions to v4

The CI workflow was using .NET 6.0.x SDK and outdated action versions,
causing build failures with .NET 9.0 target framework and 10.x packages.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

.github/workflows/aspnetcore.yml

@@ -8,11 +8,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v4
     - name: Setup .NET Core
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '6.0.x'
+        dotnet-version: '9.0.x'
     - name: Install dependencies
       run: dotnet restore
     - name: Build