Mosquitto: major update to version 2.0

Mosquitto: major update to version 2.0

Major release, full release notes available at:
https://mosquitto.org/blog/2020/12/version-2-0-0-released/

Tested on ath79, ssl full, ssl trimmed and non-ssl builds, and also all
at the same time.
Users may/will run into some of the changes in default
authentication.  Note
https://mosquitto.org/documentation/migrating-to-2-0/ for tips on
migrating.

Patch included has already merged upstream, but didn't make the release:
https://github.com/eclipse/mosquitto/pull/1930

Signed-off-by: Karl Palsson <karlp@etactica.com>