CMake: Update the README regarding the minimum cmake version required

The minimum required CMake version to build Qt on any platform is
3.16.0 at the moment. That's the first version that ships the
implementation-specific AutogenInfo.json files, which we parse
to implement the Qt moc --collect-json functionality.

Change-Id: I6160cd74fa228aa330f4c1e512ffb766afc5f8a3
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

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