Update to the retention period of safety related documentation upon equipment disposal.

A query has been raised regarding the retention period required for safety related documentation upon equipment disposal. Within SMP12 section 12.2.10.1 this retention period was stated as ten years. After review of Defence policy and regulations this retention period is stated as five years. To provide consistency with Defence policy and regulations SMP12, section 12.2.10.1 has been updated to state five years.