migrate to fern 0.7 - fern 0.6 with "colored" feature and custom allocator has potential soundness issues

[decathorpe]

fern <0.7 with the "colored" feature enabled and a custom global allocator (as used in this project) is insecure:
https://github.com/daboross/fern/blob/fern-0.6.2/README.md

It looks like aw-server only uses a custom allocator on Linux, and this issue only affects Windows:
https://rustsec.org/advisories/RUSTSEC-2021-0145.html

But it might still be a good idea to update the fern dependency to 0.7 to avoid depending on the unmaintained atty crate.