Update the MIME type of MessagePack

[kkebo]

The MIME type of MessagePack is now registered as application/vnd.msgpack.

https://github.com/msgpack/msgpack/issues/194#issuecomment-2111445791

This create is still using application/msgpack.

https://github.com/hyperium/mime/blob/938484de95445a2af931515d2b7252612c575da7/src/constants.rs#L109