Adopt consistent approach for defining InternalsVisibleTo in the repo

[captainsafia]

This is a follow-up to some PR feedback I received in #41238 (comment) (and other PRs).

We use two different strategies for defining IVT in the repo at the moment:

• Property in the build
• Assembly attribute in source

For consistency, we might want to consider adopting one standard throughout the repo and using that everywhere.

[Tratcher]

Declaring it in the csproj file is simpler and cleaner. We used the assembly attribute before because the csproj option wasn't available.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.