Improve our SQL Server client-generated GUIDs

There's a [report](https://dev.to/connerphillis/sequential-guids-in-entity-framework-core-might-not-be-sequential-3408) 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 https://github.com/dotnet/efcore/issues/30753#issuecomment-2027429203, 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](https://learn.microsoft.com/en-us/sql/t-sql/functions/newsequentialid-transact-sql?view=sql-server-ver16), that would be a good improvement that should also maintain EF backwards compatibility.