Use .NET Core 3.0 SDK

Author: mayuki

.circleci/config.yml

@@ -2,7 +2,7 @@ version: 2.1
 executors:
   dotnet:
     docker:
-      - image: mcr.microsoft.com/dotnet/core/sdk:2.2
+      - image: mcr.microsoft.com/dotnet/core/sdk:3.0
     environment:
       DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true
       NUGET_XMLDOC_MODE: skip