Upgrade ArduinoJson 6 to 7

[proddy]

ArduinoJson 7 is out. Would be interesting to test both overall performance and memory usage. Now that it uses dynamic memory allocation I'm curious what happens with mem fragmentation (Max Alloc).

https://arduinojson.org/v7/how-to/upgrade-from-v6/

[proddy]

Added to 3.6.5-dev.8

[proddy]

I also removed all the & references from JsonObject and JsonArray as there are already semantically references to JsonDocument. And added JsonObjectConst where necessary. It doesn't affect the code size or performance, perhaps prevent dangling references if they ever happen.