CMake: Bump min required CMake version for shared Qt builds to 3.16

Building a shared library Qt configuration already required a minimum
CMake version of 3.16, because we depend on CMake's Autogen.json files
which are used for metatype.json file generation.

If a lower version was used, a FATAL_ERROR was issued in
qt6_extract_metatypes.

This change is essentially moving the check to happen earlier in the
qtbase configure process.

User projects will now /also/ officially require a minimum of 3.16
(up from 3.14).

As a consequence, the min/max version policy range that is set in
the public QtFooConfig.cmake files is changed
from 3.14..3.19
to   3.16..3.20

The upper bound is raised because building and using Qt works fine
with all CMake 3.20 policies set to NEW.

[ChangeLog][CMake] Building Qt as shared libraries now requires
CMake version 3.16 or later. Building user projects with CMake using
that Qt installation also requires a CMake version of 3.16 or later.

Pick-to: 6.2
Task-number: QTBUG-95018
Change-Id: I77d2829370f1dfc90b4071bebc8a3ade654e59e6
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Craig Scott <craig.scott@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

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