shell: modules: kernel_service: deprecate log_level

Log_level command is a duplicate of 'log enable' command from
the logging command set (enabled by CONFIG_LOG_CMDS=y). Adding
warning about deprecation as 'log enable' is recommended since
it has more features (e.g. autocompletion).

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>