Remove references to SimpleMessageStreams in Orleans.Streams docs

[ReubenBond]

Issue description

Orleans 7.0 removed SimpleMessageStreams and replaced it with a separate abstraction called BroadcastChannel.
For the Streams docs, we should replace usage of SimpleMessageStreams with MemoryStreams.

For more information, see: https://learn.microsoft.com/en-us/dotnet/orleans/whats-new-in-orleans#replacement-of-simplemessagestreams-with-broadcastchannel

Separately, we need to add documentation for BroadcastChannel.
Example doc referring to SimpleMessageStream: https://learn.microsoft.com/en-us/dotnet/orleans/streaming/streams-quick-start

cc @IEvangelist @bradygaster @benjaminpetit

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Associated WorkItem - 46073