add .NET 6.x to install on ci

Author: Mpdreamz

.github/workflows/ci.yml

@@ -30,6 +30,8 @@ jobs:
     - uses: actions/setup-dotnet@v3
       with:
           global-json-file: global.json
+          dotnet-version: |
+              6.x
       env:
         NUGET_AUTH_TOKEN: ${{secrets.GITHUB_TOKEN}}