ENH Update code to reflect changes in silverstripe/admin (#1509) #1014

Job	Run time
CI / Generate matrix	11s
CI / Context	1s
CI / Check governance	1s
CI / 8.3 prf-low mariadb phpunit all	3m 12s
CI / 8.3 mysql80 phpunit all	2m 38s
CI / 8.3 mysql84 phpunit all	2m 52s
CI / 8.3 mysql80 phplinting	2m 5s
CI / 8.3 mysql80 endtoend root job1	8m 42s
CI / 8.3 mysql80 endtoend root job2	9m 13s
CI / 8.3 mysql80 endtoend root job3	9m 34s
CI / 8.3 mysql80 js	4m 20s
CI / Dispatch tag patch release	5s
	42m 54s

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