Update dotnet.yml with new build actions and .NET 8

Author: dhelper

.github/workflows/dotnet.yml

@@ -19,11 +19,13 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Setup .NET
-      uses: actions/setup-dotnet@v3
+      uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 6.0.x
+        dotnet-version: |
+          6.0.x
+          8.0.x
 
     - name: Build
       run: find dotnet-test-samples -name "*.sln" -exec dotnet build {} \;