[Development] Bumping the minimum CMake version for Apple platforms to 3.21 for Qt 6.6

[Tor Arne Vestbø]

Hi all,

We currently require CMake 3.16 for shared builds, and 3.21 for static builds. CMake 3.16 unfortunately doesn't have a feature we rely on on Apple platforms — finalizers — requiring us to maintain and take into account additional APIs for users to manually finalize their projects, with caveats and sharp corners.

We’d like to avoid the maintenance overhead of this manual finalization API going forward, and are proposing to bump the required CMake version for user projects (and Qt in effect) on Apple platforms to 3.21 for 6.6, the same version we require for static builds for all Qt platforms.

This should not present much problems for users on these platforms (macOS), as CMake is not shipped with the system, but instead installed via e.g. Homebrew, which already ships a recent version, and also makes it easy to upgrade. Our own binary installers also ship 3.24, so we’re good there as well.

As qt_finalize_target is already a noop for CMake 3.21 the work involved to bump the requirement is primarily updating documentation. 

Let us know if this causes concern.

Cheers,
Tor Arne