Upgrade to version 6.8 of Qt
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Before making a release with the new Qt version, we should:
- make sure that #1837 is fixed;
- check whether the modality problems described in https://git.jami.net/savoirfairelinux/jami-client-qt/-/issues/1855 have been resolved, and if so, revert the hack (https://review.jami.net/c/jami-client-qt/+/30403) that was implemented to fix #1905 (closed).