Platform dependency change detection

### Summary

Detecting dependency changes when they are first made to source code allows us to capture right away when such changes are being introduced. This allows a workflow which can potentially block incompatible changes (see https://github.com/dotnet/runtime/issues/38755) or provide a developer experience whereby they can easily update the platform dependency data so that the dependency is appropriately tracked.

> Note: This is a proposed experience for .NET 6 and not yet committed.

### Goals

* .NET contributors can follow a well-defined process when they make changes to the platform dependencies.
* .NET contributors can be made aware when they've made a change that affects .NET's runtime or build-time platform dependencies.
* .NET contributors can easily update .NET's required package version for a platform dependency.
* .NET contributors can be made aware when they've added a platform dependency that cannot be satisfied by a supported OS.
* Platform dependency stakeholders are made aware of changes to the dependencies.