There's no reason at all for this to be a string or to accept arbitrary
values - just because it's displayed as strings in the spec doesn't mean
we have to do the same :^)
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
