It's mildly annoying to read the current license headers, which grant
permission to use the library under the terms of the GNU Lesser General
Public License version 2 or later, but there is no such license (as it
was the GNU Library General Public License way back then). So upgrade
from version 2 to version 2.1. This is what we have specified in the
meson.build anyway.

This should almost certainly have no effect on anything.