Silverstripe v6 support #158

Job	Run time
ci / Generate matrix	11s
ci / Context	3s
ci / Check governance	3s
ci / 8.3 mysql80 phpunit all	2m 23s
ci / 8.4 mysql84 phpunit all	2m 22s
ci / 8.3 prf-low mariadb phpunit all	2m 16s
ci / 8.3 mysql80 phplinting	2m 5s
ci / Dispatch tag patch release	0s
	9m 23s

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