Update references to GNU-family licenses in spec

[swinslow]

In connection with #6 to update the License List Appendix to the latest version, any references to the GNU family of licenses (GPL, LGPL, AGPL, GFDL) that are used in the spec should be updated to the new ID format for these licenses.

Old format: GPL-2.0; GPL-2.0+
New format: GPL-2.0-only; GPL-2.0-or-later

[wking]

#37 (still in flight) does this for the license expression appendix.

[kestewart]

Yes, need a full scrub of 2.2 to update to latest GPL notation from license list.

[iamwillbar]

In the license expression specification we say "An SPDX License List Short Form Identifier with a unary"+" operator suffix to represent the current version of the license or any later version. For example: GPL-2.0+", is this deprecated and use of the -or-later license ID is preferred? Or is this still valid since it can be applied to any license?

[goneall]

GPL-2.0+", is this deprecated and use of the -or-later license ID is preferred?

-or-later license ID is preferred by the license steward.

This has a bit of complex history - although a bit out of date, this wiki page can give you a bit of background: https://wiki.spdx.org/view/Legal_Team/or-later-vs-unclear-disambiguation