Update MessagePack-CSharp (#43474)
Update MessagePack-CSharp (#43474)

6caf2996c82d2b91528fad41e9c78e09770e73d4 -> fe9fa0834d18492eb229ff2923024af2c87553f8

https://github.com/aspnet/MessagePack-CSharp/pull/2
aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore,aspnet/AspNetCore