Currently this script relies on hard-coded values specify minimum
iOS version to link against.
This patch unifies the hard-coded values into a single variable.
No functional changes.
Pick-to: 6.9 6.8
Change-Id: I12bddeb53f0fe3388ea55c841a9864722a1d5f82
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>