Set version to 6.21.3 #772

Job	Run time
Lint	17s
GCC (5)	3m 32s
GCC (6)	3m 48s
GCC (7, -fsanitize=leak -fno-sanitize-recover=all)	3m 24s
GCC (8, -fsanitize=undefined -fno-sanitize-recover=all)	5m 43s
GCC (9, -fsanitize=address -fno-sanitize-recover=all)	4m 14s
GCC (10, -funsigned-char)	3m 24s
GCC (11)	4m 49s
Clang (3.8, -stdlib=libc++)	3m 18s
Clang (3.9, -stdlib=libc++)	3m 34s
Clang (4.0, -stdlib=libc++)	3m 30s
Clang (5.0)	3m 0s
Clang (6.0)	3m 20s
Clang (7)	3m 8s
Clang (8, -fsanitize=leak -fno-sanitize-recover=all)	3m 17s
Clang (9, -fsanitize=undefined -fno-sanitize-recover=all)	5m 1s
Clang (10, -fsanitize=address -fno-sanitize-recover=all)	4m 57s
XCode (11.7)	4m 5s
XCode (12.4)	4m 34s
XCode (13.2.1)	4m 40s
Clang-Tidy	5m 4s
Test configuration on Windows	45s
Test configuration on Linux	43s
PlatformIO (atmelavr, leonardo, SD, Ethernet, avr)	35s
PlatformIO (espressif8266, huzzah, esp8266)	3m 32s
PlatformIO (espressif32, esp32dev, Ethernet, esp8266)	2m 1s
PlatformIO (atmelsam, mkr1000USB, SD, Ethernet, esp8266)	57s
PlatformIO (teensy, teensy31, esp8266)	1m 59s
PlatformIO (ststm32, adafruit_feather_f405, SD, Ethernet, esp8266)	3m 27s
PlatformIO (nordicnrf52, adafruit_feather_nrf52840, SD, Ethernet, esp8266)	2m 34s
Arduino (arduino:avr, arduino:avr:uno)	38s
Arduino (arduino:samd, arduino:samd:mkr1000)	1m 10s
Amalgamate ArduinoJson.h	5s
Coverage	3m 19s
Amalgamate ArduinoJson.hpp	5s
GCC for ARM processor	3m 53s
ESP-IDF	2m 42s
Particle (argon)	30s
Valgrind	2m 48s
CodeQL	15m 13s
	2h 7m 35s

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