Wait for Debian to update to 2.0.0.
We can monitor the mosquitto version in the debian stable repository here. As mosquitto 2.0 is in the debian testing repository already, 2.0 is expected to be in Debian 11, circa June or August 2021.
Ubuntu has it in Hirsute Hippo, due out end of April, should anyone install it on Ubuntu in the mean time.
Diff:
Currently Ubuntu (Hirsute Hippo) is shipping with Mosquitto v2. Debian still uses V1.x.
A good alternative for the documentation might be: HiveMQ Cloud. They offer a free tier for 100 devices but with no SLA. One advantage would be that you would not have to install linux, but users who want a free and open source on-premises solution may need to look elsewhere.