Lower the CMake minimum required version for single-config builds

Ubuntu 20.04 is a supported platform and comes with CMake 3.16. The
current state of the Qt build works fine with this CMake version, mainly
because the default build on Linux is single-config.

For multi-config builds we have higher version requirements as
documented in cmake/README.md.

This lowers the barrier for users using single-config builds who don't
want or cannot upgrade to CMake >= 3.18.

For Ninja Multi-Config builds, which were introduced by CMake 3.17, we
yield a warning that 3.18.3 is the lowest supported version.

Change-Id: Ia918b9f2b494508e86301ffc0e138d3ad4dbaf86
Reviewed-by: Craig Scott <craig.scott@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit 20a17911576297f22b9da9849b677476c2d6e0ea)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

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