For reasons unknown to me, COPYING previously contained the full text of
LGPLv2 (not 2.1!) despite the entirety of this project being under GPL
v2 or later. It also contained a preamble that the license is >= 2, not
== 2.

Make COPYING a verbatim copy of GPL v2, and add the "or later" note to
README. This appears to be standard for similarly-licensed GNOME
projects I checked (including gnome-shell and mutter). This will also
cause GitLab to display the correct license, rather than "This project
is licensed under the Other." with a broken link.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-initial-setup/-/merge_requests/135>