Add a global.json that requires 2.0.0 of the SDK.  Projects fail to build with 2.1.
Add a global.json that requires 2.0.0 of the SDK.  Projects fail to build with 2.1.

GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples