Historically, libnm claimed to be LGPL-2.0+ licensed. But as keep mixing that
with LGPL-2.1+ code (e.g. from systemd), so probably even the parts that
claimed to be LGPL-2.0+, were not entirely correctly doing so.
Anyway, since we switched to SPDX license identifiers, we now claim
everywhere that the right license is LGPL-2.1+. Update the comment to
reflect that.
Also drop the paragraph about "autoconf mechanism". The general
guideline is self evident, while it didn't mention meson builds.
Also, a first time contributor likely won't likely be concerned about
this, as this is already advanced.
