* Introduce a new signature to qt6_wrap_cpp so that it accepts a target
name as the first parameter.
* Update the qt_wrap_cpp documentation.
* Show a warning message when the previous signature is used.
* Amends dc4159286b because it deleted
the `qt_wrap_cpp_3` example by mistake.
[ChangeLog][Build System] A new target-based signature was added for
qt_wrap_cpp. The usage of the previous output-variable signature is
deprecated and will issue a warning.
Fixes: QTBUG-126460
Change-Id: I85b7c516d6aebd8bbfda441fb6c68999e4898479
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
(cherry picked from commit f3d29dfb89f3e35623edd7e5bca934a1e85a6c35)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
a9dbdcd986