Upgrade to SilverStripe 6 #33

Job	Run time
CI / Context	4s
CI / Generate matrix	17s
CI / 8.3 prf-low mariadb phpunit all	2m 31s
CI / 8.4 mysql84 phpunit all	1m 35s
CI / 8.3 mysql80 phpunit all	1m 13s
CI / 8.3 mysql80 phplinting	1m 20s
CI / 8.3 mysql80 phpcoverage	2m 1s
CI / Check governance	0s
CI / Dispatch tag patch release	0s
	9m 1s

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