Upgrade MessagePack to v3+ #305
Description
Upgrade MessagePack to v3 to take advantage of source code generators.
See release notes here: https://github.com/MessagePack-CSharp/MessagePack-CSharp/releases/tag/v3.0.3 and https://neuecc.medium.com/messagepack-for-c-v3-release-with-source-generator-support-893ed30d0e89