Add W3C license to threat model copyright.txt and LICENSE files

[eric-schuh]

This needs to be a W3C License.

Originally posted by @msporny in #655

[BigBlueHat]

Because this is documentation/specification text this should likely use this W3C License (vs. the Apache-2.0): https://www.w3.org/copyright/software-license-2023/

[eric-schuh]

Joe and I had a discussion about this and concluded that both the LICENSE and copyright.txt files in the threat_model folder should just be removed.