CMake: Provide the minimum darwin deployment target needed by Qt

This is useful information for people that want to use Qt in their
project with CMake.

Unfortunately there is no good way to actually set
CMAKE_OSX_DEPLOYMENT_TARGET automatically with the Qt provided value,
because it needs to be set before the first project() call in a
CMake project.

This means it can be set either on the command line, or manually by the
application developer in CMakeLists.txt before the first project()
call or via a CMake toolchain.

In Qt 6 we provide a CMake toolchain file with the deployment target
already set, so if the application developer uses the Qt generated
toolchain, they will get the value automatically.

In Qt 5 the value can only be read for informational purposes only,
from the QT_DARWIN_MIN_DEPLOYMENT_TARGET property on the
Qt5::Core target. The same property is set in Qt 6 for compatibility
resons.

Task-number: QTBUG-76167
Task-number: QTCREATORBUG-22466
Pick-to: 5.15
Change-Id: Id61d70dc9a1b7a0e7cdb90b1e679171ea178126b
Reviewed-by: Joerg Bornemann <[email protected]>
Reviewed-by: Eike Ziller <[email protected]>

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