updating umap-sharp

[vpenades]

Hi, I've found your most interesting umap project, and I would like to use it in some experiments.

Reviewing the project, though, I found it's a bit outdated, so I would consider opening a PR, provided you're fine with reviewing the changes and eventually publishing a new package.

The changes I would like to do don't fundamentally change anything of the project, and I can summarize as:

• bumping frameworks to net.8
• bringing package references up to date
• replacing some float[] by ReadOnlyMemory<float> (keeping the public API intact) this is safer since after all, the embedding vectors are expected to be immutable
• replace SIMD operations by System.TensorPrimitives, which is the new, and now recomended (and faster) approach for vector math streaming.