Update AutoRest C# version (Azure#17516)

Update AutoRest C# version to 3.0.0-beta.20201217.1

Author: azure-sdk

eng/Packages.Data.props

@@ -117,7 +117,7 @@
     All should have PrivateAssets="All" set so they don't become pacakge dependencies
   -->
   <ItemGroup>
-    <PackageReference Update="AutoRest.CSharp.V3" Version="1.0.0-alpha.20201203.2" PrivateAssets="All" />
+    <PackageReference Update="AutoRest.CSharp.V3" Version="3.0.0-beta.20201217.1" PrivateAssets="All" />
     <PackageReference Update="Azure.ClientSdk.Analyzers" Version="0.1.1-dev.20200929.2" PrivateAssets="All" />
     <PackageReference Update="coverlet.collector" Version="1.3.0" PrivateAssets="All" />
     <PackageReference Update="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="3.3.1" PrivateAssets="All" />