The mail you received in unrelated to Airwave. It is warning that a Central subscription (which seems to be an eval as that has 10 devices, for 90 days) is about to expire. Probably someone requested that evaluation. If you are sure that you don't use Aruba Central, but Airwave, you can ignore that message.
It may make sense to count back 90 days before July 28th (which is I believe somewhere last days of April or first days of May), and see if you can find who may have requested the Central evaluation.
As a double check, validate in Airwave that you do have those 15 licenses and they are permanent. In that case, at least Airwave will continue to run after July 28th.