Bump up build sdk

Author: macsux

.github/workflows/dotnet.yaml

@@ -14,7 +14,7 @@ jobs:
       - name: Setup dotnet
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: 2.2.108
+          dotnet-version: 3.1.200
       - run: dotnet build --configuration Release
       - name: test with dotnet
         run: dotnet test /p:CollectCoverage=true /p:ExcludeByFile=\"**/KubernetesClient/generated/**/*.cs\" /p:CoverletOutputFormat="cobertura"