API Replace Extension subclasses #603

Job	Run time
ci / Generate matrix	38s
ci / Context	1s
ci / Check governance	0s
ci / 8.1 prf-low mysql57 phpunit all	5m 57s
ci / 8.2 mariadb phpunit all	5m 2s
ci / 8.3 mysql80 phpunit all	7m 28s
ci / 8.1 mysql57 phplinting	1m 40s
ci / 8.1 mysql57 endtoend root job1	5m 18s
ci / 8.3 mysql80 endtoend root job1	6m 28s
ci / 8.1 mysql57 endtoend root job2	8m 7s
ci / 8.3 mysql80 endtoend root job2	8m 51s
ci / 8.1 mysql57 js	4m 58s
ci / 8.1 mysql57 doclinting	2m 14s
ci / Dispatch tag patch release	0s
	56m 42s

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