Target .net 8.0 in CI

The test project already targets .net 8.0, so CI needs to be updated as
well.

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
         submodules: true
     - uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: '6.0.*'
+        dotnet-version: '8.0.x'
     - run: dotnet test
       if: matrix.os == 'ubuntu-latest'
     - run: dotnet test --filter 'Category!=integration'