Remove support for DotNetCliToolReference

[dsplaisted]

Back in .NET Core 1 and 2, we had a concept called "project tools". These could be declared in the project file using a DotNetCliToolReference. These were effectively replaced by local tools in .NET Core 3, and we made it so that you could not create a project tool targeting anything higher than .NET Core 2.2.

Given that .NET Core 2.2 is long out of support, we should consider removing the code from the .NET SDK which supports creating and consuming these tools. This includes ProjectToolsCommandResolver, tasks and targets supporting creating these packages, and possibly more.

[KalleOlaviNiemitalo]

https://learn.microsoft.com/en-us/dotnet/core/additional-tools/xml-serializer-generator tells users to add both PackageReference and DotNetCliToolReference. Doesn't Microsoft.XmlSerializer.Generator require DotNetCliToolReference any longer?