OPENTHREAD_CONFIG_IP6_SLAAC_DEPRECATION_INTERVAL macro

@def OPENTHREAD_CONFIG_IP6_SLAAC_DEPRECATION_INTERVAL Specifies the deprecating time of SLAAC addresses in seconds. Applicable only if OPENTHREAD_CONFIG_IP6_SLAAC_ENABLE is enabled. Deprecating interval is used once an on-mesh prefix is removed from Network Data before removing the SLAAC address.