Bump dependency versions and fix deprecation warnings
Update library dependencies for Arduino JSON (6.21.2) and ESP-FlexyStepper (>1.4.7)
Fixed some deprecation warnings in ArduinoJSON
NOTE: in case you experience issues with latest espressif toolchain and some warnings about SHA1 not being defined when you try to compile the code, then you might need to install the latest HEAD revision from Master branch of ESP Async WebServer (https://github.com/me-no-dev/ESPAsyncWebServer.git) since at this point of time the version 1.2.3 in Platformio registry is not up2date.
You can fix this in Platformio by adding the dependency in your platformio.ini (e.g.):
lib_deps = ESP-FlexyStepper@^1.4.7, https://github.com/me-no-dev/ESPAsyncWebServer.git