Migrate to latest mbed-os version

[thendrickson]

I attempted the migrate and quickly ran into set_session() having gone away

[RafaellaN]

I was looking the set_session() too, did they change it to something else? Did you found anything about it? I wanted to see If there is a get_session() to get the frame counters..

[janjongboom]

@RafaellaN We were working on a better implementation of set_session() in the core of the LoRaWAN stack (it was hacked on in a fork for FUOTA to work), but this never landed in Mbed OS. Since then LoRaWAN seems to be deprioritized in Mbed OS, and both me and @hasnainvirk (who worked on this) have left Arm. So I'm not expecting this to be updated.