[Proposal]: Show obsolete errors from overrides

# Show obsolete errors from overrides

* Specification: None yet.
* Discussion: https://github.com/dotnet/csharplang/discussions/8937

## Summary
[summary]: #summary

Today, when an override of a virtual member adds `Obsolete`, we do not warn that the method is obsoleted when the derived type is statically known as the receiver. For example:

```cs
abstract class Base
{
public abstract void M();
}

class Derived : Base
{
[Obsolete]
public override void M() => throw null;

public void M2()
{
M(); // No warning
}
}
```

This is something that we can detect at warn about.

## Design meetings