Recommend using SPDX controlled vocabulary for licenses

[alanorth]

CG Core v2 currently gives conflicting advice for licenses: we suggest using SPDX, but give an example of a URL to a license page. I suggest we change this to recommend using SPDX License Identifiers rather than URLs. This is what CGSpace, MELSpace, and WorldFish's Knowledge Archive have been doing for several years already.

The SPDX License Identifiers are short, meaningful, and safe to use in any programming language (no spaces or reserved characters). Furthermore, SPDX was just recently published as ISO/IEC 5962:2021 so there is even more industry recognition of this vocabulary.