APICompat falsely reports a compatibility problem when an API moves from virtual to override

[ericstj]

Describe the bug

A method may transition from being virtual itself to overriding a method on it's base type when the base type adds a virtual method of the same name.

This is a compatible change, but API flags the method that changes to override.

To Reproduce

Build https://github.com/ericstj/efcore/tree/apiCompatSpike-8.0

Observe the following suppressions were needed:
ericstj/efcore@677d319#diff-742b3f6f609d60660f78e22b813208458dfbbfc1ff4f1f62315b817bd1966dc6R4-R15

Those are representing these differences between netstandard2.0 and netstandard2.1 due to the additions of Span API to stream.
https://github.com/dotnet/efcore/blob/d4fd971bf18a0fc786341bdc3588302f2babf264/src/Microsoft.Data.Sqlite.Core/SqliteBlob.cs#L187-L189
https://github.com/dotnet/efcore/blob/d4fd971bf18a0fc786341bdc3588302f2babf264/src/Microsoft.Data.Sqlite.Core/SqliteBlob.cs#L261-L263

Those differences are compatible and should not be flagged.

[ghost]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage