Update setup-dotnet to use 8.0.x

Author: Heliozoa

.github/workflows/test.yml

@@ -80,7 +80,7 @@ jobs:
       - name: Install C#
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: "6.0.x"
+          dotnet-version: "8.0.x"
 
       - name: Install R
         if: matrix.os == 'ubuntu-latest'