License field: use SPDX ID when translating to citation files

[florisvdh]

For example, GPL (>= 3) in DESCRIPTION should translate to GPL-3.0-or-later, both for Zenodo and for the cff format; the string is not to be copied as such (currently the case).

See https://spdx.dev/ids/ and the Zenodo API & CFF docs. Also, https://blog.zenodo.org/2018/11/22/2018-11-22-new-licenses/.

[ThierryO]

Note that checklist currently only allows the GPL-3 or MIT licenses for packages, not GPL (>=3).

[florisvdh]

Some alignment may be considered with usethis for GPL; usethis::use_gpl3_license() implements GPL (>=3).

Documentation of the include_future argument (default TRUE):

If TRUE, will license your package under the current and any potential future versions of the license. This is generally considered to be good practice because it means your package will automatically include "bug" fixes in licenses.