This commit adds four patches to fix various issues that were preventing
Qt 6.6.1 from building on Debian testing and Debian unstable. These
patches are backports of fixes that were applied in later versions of
Qt; links to the relevant commits are included in the description at the
top of each patch.

There are also two changes in the Dockerfiles for Debian testing and
Debian unstable:
1) CMake 3.21 is no longer installed via the install-cmake.sh script.
   This is not necessary anymore given that Debian testing and Debian
   unstable now both come with more recent versions of CMake by default
   (3.29 and 3.30 respectively).
2) The libre2-dev package (which is part of the dependencies in
   debian-qt/control) is removed from the Docker image in order to force
   Qt to build using the bundled version of the RE2 library. This is
   necessary because the system version of the library on testing and
   unstable (libre2-11) is not compatible with earlier versions due to
   an API change, c.f.
   https://codereview.qt-project.org/c/qt/qtwebengine/+/516094

GitLab: #1822
Change-Id: I763fb6692949052e2a846b3f4ce54619e6d98108