Reconsider Moq usage because of privacy issues

[lachmatt]

Reconsider usage of Moq because of privacy issues described https://github.com/moq/moq/issues/1372.
For reference, Moq was replaced with NSubstitute in open-telemetry/opentelemetry-dotnet-instrumentation#2816

[Kielek]

Notes from August 15 sig meeting:

• No action necessary today since the version we use is prior to Moq adding SourceLink dependency.
• SourceLink has been removed in the latest release suggesting the maintainer is responsive to the community https://github.com/moq/moq/releases/tag/v4.20.69.
• If we do consider removing our use of Moq, we should consider attempting to eliminate using a mock library completely instead of replacing it with an alternative. Potentially a “good first issue/help wanted”.

[cijothomas]

#2534 this issue is related. It has links to multiple PRs removing MoQ with InMemoryExporter.