Silverstripe 6 compatibility (#79) #304

Job	Run time
ci / Generate matrix	16s
ci / Context	2s
ci / Check governance	2s
ci / 8.3 prf-low mariadb phpunit all	3m 39s
ci / 8.3 mysql80 js	4m 44s
ci / 8.3 mysql80 phpunit all	3m 47s
ci / 8.3 mysql80 phplinting	1m 55s
ci / 8.4 mysql84 phpunit all	3m 36s
ci / Dispatch tag patch release	0s
	18m 1s

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