update license data and convert to SPDX format
The license data was gathered from the 2.38.1 tarball. The licensecheck
tool was run:
find -type f -regextype egrep ! -regex '^(Documentation/.*\.txt$|(t/(chainlint|perf/p[0-9]{4}|t[0-9]{4}).*))' \
-exec licensecheck --shortname-scheme spdx {} + | LANG=C sort >licensecheck
The contents were reviewed, removing files which are not shipped or were
UNKNOWN to licensecheck. Of the UNKNOWN files, most lacked a specific
license header and are thus treated as GPL-2.0-only. The code in
reftable/ is licensed as BSD 3-Clause per reftable/LICENSE.