Use already defined generateAssemblyReferenceSource rather defining a new target

[Anipik]

Fixes #948
Fixes #34604

[ghost]

Tagging subscribers to this area: @ViktorHofer
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[Anipik]

c:\git\runtime\src\libraries\System.Runtime\src>dotnet msbuild /t:GenerateReferenceAssemblySource /bl
Microsoft (R) Build Engine version 16.7.0-preview-20229-03+2cee6d020 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

C:\Program Files\dotnet\sdk\5.0.100-preview.5.20254.3\MSBuild.dll -distributedlogger:Microsoft.DotNet.Tools.MSBuild.MSBuildLogger,C:\Program Files\dotnet\sdk\5.0.100-preview.5.20254.3\dotnet.dll*Microsoft.DotNet.Tools.MSBuild.MSBuildForwardingLogger,C:\Program Files\dotnet\sdk\5.0.100-preview.5.20254.3\dotnet.dll -maxcpucount -verbosity:m /bl /t:GenerateReferenceAssemblySource .\System.Runtime.csproj
  Generated reference assembly source code: c:\git\runtime\src\libraries\System.Runtime\ref\System.Runtime.cs

[Anipik]

should wait until this dotnet/arcade#5419 is avaliable in runtime

[ViktorHofer]

I don't want to be over-aggressive with the batched rollout but you are renaming a target that devs are used to. That's a candidate for the next batched rollout: #35895.

[ericstj]

You can keep the old target and just have it depend on the new one. If you want to be cute you could emit a warning in the old target about deprecation. Then delete the old one in next batched rollout.

[ViktorHofer]

I like cute.

[safern]

You can keep the old target and just have it depend on the new one. If you want to be cute you could emit a warning in the old target about deprecation. Then delete the old one in next batched rollout.

I like this idea to not block until next batch rollout.

[Anipik]

i added the target back to unblock this and also updated the genapi version. I have tested the change locally and both the targets work