generalize GPL exception

we have explicit approval from:
- Anton Khirnov <anton@khirnov.net>
- Jeremy Katz <katzj@fedoraproject.org>
- Jesse Weaver <pianohacker@gmail.com>
- Marc Hoersken <info@marc-hoersken.de>
- Michael J Gruber <michael@grubix.eu>
- Noa Resare <noa@resare.com> (formerly Daniel)
- Oliver Runge <oliver.runge@gmail.com>
- Patrick Steinhardt <ps@pks.im>
- Theodore Ts'o <tytso@mit.edu>

notably missing approval from:
- Michael Elkins <me@sigpipe.org>

further missing approval from:
- Eivind Eklund <eivind@FreeBSD.org>
- Georgy Kibardin <georgy@kibardin.name>
- Jack Stone <jwjstone@fastmail.fm>
- Jan Synacek <jsynacek@redhat.com>

still, because
- isync already contains an exception for OpenSSL,
- every contributor implicitly agreed to that exception, and
- that exception exists specifically because of the advertising clause
  in OpenSSL < v3's license,
i'm assuming that the MIA contributors are actually fine with the
proposed change.

LICENSES/LicenseRef-isync-GPL-exception.txt

@@ -8,5 +8,5 @@ Usage-Guide:
     SPDX-License-Identifier: <SPDX-License> WITH LicenseRef-isync-GPL-exception
 License-Text:
 
-As a special exception, mbsync may be linked with the OpenSSL library,
-despite that library's more restrictive license.
+As a special exception, mbsync may be linked with libraries with a more
+restrictive license if the only incompatibility are advertising clauses.