Update dotnet.yml

Author: stephanstapel

.github/workflows/dotnet.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v2
       with:
-        dotnet-version: 6.0.x
+        dotnet-version: 8.0.x
     - name: Restore dependencies test
       run: dotnet restore ZUGFeRD-Test/ZUGFeRD-Test.csproj
     - name: Build test