Zstandard compression key in the code must be kept in sync with the spec

This enum member https://github.com/mongodb/mongo-csharp-driver/blob/master/src/MongoDB.Driver.Core/Core/Compression/ICompressor.cs#L42 should be kept in sync with the spec https://github.com/mongodb/specifications/blob/master/source/compression/OP_COMPRESSED.rst#mongodb-handshake-amendment . So, it should be replaced on `zstd`