.Net: [MEVD] Remove the ability to use doubles (float64) as embedding

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Some of our connectors (Weaviate, Mongo+Azure Mongo, Azure NoSQL, Redis JSON) support ReadOnlyMemory<double> as embedding types. No actual major embedding model supports producing 64-bit floating point numbers AFAIK, and there's apparently no reason for them to.

I'm assuming that doubles were supported in JSON-based connectors, where the number isn't actually typed (in the sense that the number representation is textual in JSON). However, this does not mean we should be exposing double embeddings to users: it's likely that most vector databases will truncate the precision anyway, and this isn't an actual supported scenario anywhere.

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Note: do only after #10492 is done, to avoid conflicts.