cmake: Use CMake DEPRECATION level instead of WARNING

CMake provides a DEPRECATION level for messages.
This should be used instead of WARNING for deprecated features.

Using DEPRECATION allows users to disable the deprecation warning with
`cmake -Wno-deprecated ...`
or turn deprecation warning into an error using
`cmake -Werror=deprecated`

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>