use Sdks attribute in dotnet new templates (#4916)

* Move dotnet-new templates to Sdk attribute

* Update to MSBuild 15.1.0-preview-000454-01

To pick up a fix for Microsoft/msbuild#1431.

* Fix template newlines

* Fix casing on Microsoft.Net.Sdk

* Move migration test csproj's to Sdk attribute

* Disable parallel sdk restore

Each SDK restore operation will try to manipulate the same assets.json file since the dependency name&version are injected into a common csproj file. This can cause runtime failures when two NuGets try to restore the project at once.

* Make casing of SDK 'NET' and not 'Net'

* Remove redundatn imports

* Fix test string

* Additional race

* Replacing the SDK with the Web.Sdk when it is a Web project.

* Fixing the test by writting the csproj before running the migration rule.

TestAssets/TestProjects/TestAppDependencyGraph/CsprojLibrary2/CsprojLibrary2.csproj

@@ -1,5 +1,4 @@
-<Project ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" />
+<Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
 
   <PropertyGroup>
@@ -14,14 +13,8 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Sdk">
-      <Version>$(CLI_NETSDK_Version)</Version>
-      <PrivateAssets>All</PrivateAssets>
-    </PackageReference>
     <PackageReference Include="NETStandard.Library">
       <Version>1.6.0</Version>
     </PackageReference>
   </ItemGroup>
-
-  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
 </Project>