Set CMake min version to 3.6, test CI with 3.6 & latest release.

[microbit-carlos]

Also minor updates to the workflow, using a newer Ubuntu image and uploading the BLE hex file as well.

Fixes https://github.com/lancaster-university/microbit-v2-samples/issues/38.

Upstream CODAL PR:

• https://github.com/lancaster-university/codal/pull/52