Build fails with stable 3.0.0 due to incompatible update in ArduinoJson

[euphi]

See https://travis-ci.org/github/euphi/Homie_BareMinimum :

Build fails for stable 3.0.0 because ArduinoJson 6.17.0 introduced an incompatible change.

Develop compiles fine again, because I already fixed this for develop in 90b8872 by changing the dependency to ArduinoJson with a new semver syntax to follow to "minor" version instead of major.
(~6.11.4 instead of ^6.11.4). This needs to be added to release-branch. However version prio to 6.17 seem to be compatible, so maybe we want to go to ~6.16.1 to avoid downgrades in recent installations?

#develop shall be changed to support latest ArduinoJson, I'll open another issue for this.