Updating dotnet to 5.0

Author: darkl

.github/workflows/dotnetcore.yml

@@ -12,7 +12,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.0.101
+        dotnet-version: 5.0
     - name: Build with dotnet
       working-directory: src/netstandard/
       run: dotnet build --configuration Release