[Project Layout] Centralize "net7.0" to a variable defined in a root `Directory.Build.props` Then use that variable to set the TargetFramework in every project. i.e:

[stephen-hawley]

          Move the "net7.0" to a variable defined in a root `Directory.Build.props` Then use that variable to set the TargetFramework in every project. i.e:

<TargetFramework>$(DOTNET_TFM)</TargetFramework>
or
<TargetFramework>$(DOTNET_TFM)-ios</TargetFramework>

Originally posted by @mcumming in #807 (comment)