Upgrade to upstream version 1.9.4, build and ship Qt6 libraries

Hefeerequested to merge
qt-kde-team/3rdparty/qxmpp:wip-1.9 into master
  • New upstream release (1.9.4).
  • Build against Qt6, update build-deps and deps with the info from cmake.
  • Adapt packaging to new Qt6 library sonames: libQXmppQt6 and libQXmppOmemoQt6.
  • Drop libqxmpp-dev transitional dummy package.
  • Drop obsolete Breaks/Replaces.
  • Bump Standards-Version to 4.7.0, no change required.
  • Review copyright information.
Edited by Aurélien COUDERC

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