API Replace Extension subclasses #934

Job	Run time
CI / Context	1s
CI / Generate matrix	24s
CI / Check governance	0s
CI / 8.3 prf-low mysql57 phpunit all	2m 51s
CI / 8.3 mariadb phpunit all	2m 13s
CI / 8.3 mysql57 phplinting	2m 18s
CI / 8.3 mysql57 endtoend root job1	9m 32s
CI / 8.3 mysql80 endtoend root job1	12m 30s
CI / 8.3 mysql57 endtoend root job2	8m 37s
CI / 8.3 mysql80 endtoend root job2	8m 34s
CI / 8.3 mysql57 endtoend root job3	7m 5s
CI / 8.3 mysql80 endtoend root job3	7m 50s
CI / 8.3 mysql57 js	5m 45s
CI / Dispatch tag patch release	0s
	1h 7m 40s

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