Added in missing SDK's for release

Author: thompson-tomo

.github/workflows/ci.yml

@@ -13,7 +13,10 @@ jobs:
       - name: Setup dotnet
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: 3.1.x
+          dotnet-version: |
+            2.1.x
+            3.1.x
+            6.0.x
       - name: Clean package cache as a temporary workaround for https://github.com/actions/setup-dotnet/issues/155
         working-directory: ./source
         run: dotnet clean -c Release && dotnet nuget locals all --clear