Eliminate the need to bootstrap new major SDK versions

### Describe the Problem

Linux distros need to bootstrap each new major versions of .NET.
Bootstrapping takes a considerable amount of time and effort, sometimes requiring special approval or overrides from distros.

### Describe the Solution

Ideally, we should be able to build new major versions of .NET using the previous major version. For example, building .NET 8.0.100 with the latest source-built version of .NET 7.0.

This imposes a significant restriction on .NET product teams, as they would not be able to use new .NET features until they are generally available in a public source-built release.

A potential solution is building a (small) bootstrap compiler against the previous .NET SDK, which is capable of building the current version of Roslyn, which can then be used to build the whole SDK.

cc @mirespace @omajid @richlander @crummel @MichaelSimons