Improve our SQL Server client-generated GUIDs

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There's a report that our current client-side sequential GUIDs generator for SQL Server doesn't work well for concurrent environments; #30753 was opened to consider switching to server-generated GUIDs instead, as a way of mitigating that.

However, @JackTrapper posted an analysis in #30753 (comment), according to which the issue could be mitigated by improving the client-side generation logic instead. If this indeed makes our client-generated GUIDs as indexing-efficient as server-side ones generated via NEWSEQUENTIALID, that would be a good improvement that should also maintain EF backwards compatibility.

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@JackTrapper would you be interested in submitting a PR for this? EF's client-side GUID generator for SQL Server is in https://github.com/dotnet/efcore/blob/main/src/EFCore/ValueGeneration/SequentialGuidValueGenerator.cs.