macOS: set minimum deployment target correctly

Fix T88419

`CMAKE_OSX_DEPLOYMENT_TARGET` is now always the same as
`OSX_MIN_DEPLOYMENT_TARGET`. I don't know the historical reason of
why they were allowed to be different.

`MACOSX_DEPLOYMENT_TARGET` has been moved to platform_apple_xcode
because it's helpful to deduce `CMAKE_OSX_DEPLOYMENT_TARGET` and
platform_apple is included after platform_apple_xcode. Although it seems
useless now that we're explicitly setting `CMAKE_OSX_DEPLOYMENT_TARGET`.
Remove it ? `man clang` says that `MACOSX_DEPLOYMENT_TARGET` is used
if `mmacosx-version-min` is not present.

Docs suggest that `CMAKE_OSX_DEPLOYMENT_TARGET` should be set before
project call. Should we do that, like in D10838, using
`blender_project_hack_pre` ?

Maniphest Tasks: T88419

Differential Revision: https://developer.blender.org/D11323