Switch to the built-in .NET Guid.CreateVersion7

Version 9.0 of the provider switched to generating version 7 Guids (#2909). However, since it suported .NET 8.0, the built-in [`Guid.CreateVersion7()`](https://learn.microsoft.com/en-us/dotnet/api/system.guid.createversion7?view=net-9.0) could not be used and a copy-paste implementation was used.

For EFCore.PG 10 we're targeting .NET 10, so we can clean up and switch to `Guid.CreateVersion7()`.