Use SPDX instead of putting the actual license

One of the few actual asks from the LF for hosting the project is for us to use the SPDX [1] licensing system, which is a more abbreviated way of specifying the license of a project. It's a system they promote. An example of it in Valkey is https://github.com/valkey-io/valkey/blob/d13aad45f4701e81fcda44976171bdeaadcb1060/src/hashtable.c#L1. 


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