Adjust vertical CRS transformation code according to recent QGIS-plugin changes

QGIS-Plugin code: https://github.com/MerginMaps/qgis-plugin/pull/884

Key changed from `SkipElevationTransformation` to `ElevationTransformationEnabled` (mind the boolean flip).
Let's update the codebase to use the new naming.

Prototype branch: `fix/vertical-crs-keys`