Update AutoRest C# version (#17643)
Update AutoRest C# version (#17643)

* Update AutoRest C# version to 3.0.0-beta.20201217.3
brjohnstmsft/azure-sdk-for-net,jackmagic313/azure-sdk-for-net,AsrOneSdk/azure-sdk-for-net,ayeletshpigelman/azure-sdk-for-net,brjohnstmsft/azure-sdk-for-net,AsrOneSdk/azure-sdk-for-net,jackmagic313/azure-sdk-for-net,brjohnstmsft/azure-sdk-for-net,ayeletshpigelman/azure-sdk-for-net,ayeletshpigelman/azure-sdk-for-net,AsrOneSdk/azure-sdk-for-net,brjohnstmsft/azure-sdk-for-net,yugangw-msft/azure-sdk-for-net,yugangw-msft/azure-sdk-for-net,AsrOneSdk/azure-sdk-for-net,ayeletshpigelman/azure-sdk-for-net,jackmagic313/azure-sdk-for-net,brjohnstmsft/azure-sdk-for-net,yugangw-msft/azure-sdk-for-net,jackmagic313/azure-sdk-for-net,jackmagic313/azure-sdk-for-net