Remove `final` from `JsonArray`, `JsonObject`, and `JsonVariant` #879

Job	Run time	Billable time
Lint	17s	0s
GCC (5)	2m 43s	0s
GCC (6)	3m 2s	0s
Clang (3.8, -stdlib=libc++)	2m 44s	0s
GCC (7, -fsanitize=leak -fno-sanitize-recover=all)	2m 35s	0s
Clang (3.9, -stdlib=libc++)	2m 53s	0s
GCC (8, -fsanitize=undefined -fno-sanitize-recover=all)	3m 27s	0s
Clang (4.0, -stdlib=libc++)	2m 52s	0s
GCC (9, -fsanitize=address -fno-sanitize-recover=all)	3m 5s	0s
Clang (5.0)	2m 37s	0s
GCC (10, -funsigned-char)	2m 44s	0s
Clang (6.0)	2m 51s	0s
GCC (11)	3m 2s	0s
Clang (7)	2m 39s	0s
Clang (8, -fsanitize=leak -fno-sanitize-recover=all)	2m 46s	0s
Clang (9, -fsanitize=undefined -fno-sanitize-recover=all)	3m 8s	0s
Clang (10, -fsanitize=address -fno-sanitize-recover=all)	3m 6s	0s
Clang-Tidy	4m 3s	0s
XCode (11.7)	4m 2s	0s
XCode (12.4)	4m 28s	0s
XCode (13.2.1)	4m 40s	0s
PlatformIO (atmelavr, leonardo, SD, Ethernet, avr)	28s	0s
PlatformIO (espressif8266, huzzah, esp8266)	2m 42s	0s
PlatformIO (espressif32, esp32dev, Ethernet, esp8266)	1m 41s	0s
Test configuration on Windows	33s	0s
PlatformIO (atmelsam, mkr1000USB, SD, Ethernet, esp8266)	47s	0s
Arduino (arduino:avr, arduino:avr:uno)	26s	0s
Test configuration on Linux	25s	0s
PlatformIO (teensy, teensy31, esp8266)	1m 19s	0s
Arduino (arduino:samd, arduino:samd:mkr1000)	48s	0s
PlatformIO (ststm32, adafruit_feather_f405, SD, Ethernet, esp8266)	2m 6s	0s
PlatformIO (nordicnrf52, adafruit_feather_nrf52840, SD, Ethernet, esp8266)	1m 34s	0s
Amalgamate ArduinoJson.h	5s	0s
GCC for ARM processor	3m 3s	0s
Coverage	2m 2s	0s
Particle (argon)	31s	0s
Valgrind	1m 45s	0s
Amalgamate ArduinoJson.hpp	4s	0s
CodeQL	8m 16s	0s
ESP-IDF	1m 40s	0s
	1h 33m 59s	0s

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