Travis: update to 20.04
Travis: update to 20.04

Updating to the latest Travis provided environment. It's been a year around and we can benefit from
more updated versions we use.

More info https://docs.travis-ci.com/user/reference/focal/

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed