CONTRIBUTING: update comment about requiring LGPL-2.1+ license instead of LGPL-2.0+
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.
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