Use trimmed-down RID graph for .NET 8 and higher

[dsplaisted]

With .NET 8, we are making some RID simplifications: dotnet/designs#260

Basically, we will no longer use RIDs for every different Linux distro and version, and will instead rely on "portable" RIDs such as linux-x64.

To support this in the SDK, we should do the following:

• Include a simplified RID graph in the .NET SDK, with contents specified here: https://gist.github.com/elinor-fung/d61f6011dff382f6299bd63ecf694a6d
• When targeting .NET 8 and higher, use the simplified RID graph for RuntimeIdentifierGraphPath by default. An MSBuld property (TBD) should switch back to the full RID graph
• Source-built versions of the SDK will use a RID specific to the corresponding distro. That RID should be included in the simplified RID graph for source-built SDKs. So there should be an MSBuild target/task that can be used to insert a specified RID in the simplified RID graph (parented to another specified RID).