Mark `GrainCancellationToken` and related symbols as obsolete in favor of `CancellationToken`

Now that Orleans supports `CancellationToken`, we should consider marking `GrainCancellationToken` as obsolete.

`GrainCancellationToken` does have more functionality than `CancellationToken` itself, largely because they are not associated with individual requests (they have globally unique identifiers). That also adds significant complexity, though.

Start by reviewing the issue's proposal to mark GrainCancellationToken and related symbols obsolete in favor of CancellationToken. Determine which related symbols are in scope and what compatibility or functionality differences must be resolved; done means the deprecation scope and replacement guidance are clearly established.

Written by the indexing model from the issue text.