CI/CD

Bump @fullhuman/postcss-purgecss from 4.1.3 to 8.0.0 #2166

Job	Run time
Test addon (floating dependencies) (ember)	52s
Test addon (locked dependencies) (test-apps)	45s
Test addon (locked dependencies) (ember)	45s
Lint files	25s
Test addon (locked dependencies) (node)	24s
Test addon (floating dependencies) (test-apps)	42s
Test addon (floating dependencies) (node)	22s
Test compatibility (ember-lts-4.12)	1m 5s
Test compatibility (embroider-safe)	1m 5s
Test compatibility (ember-beta)	44s
Test compatibility (ember-lts-4.8)	1m 1s
Test compatibility (ember-lts-5.4)	1m 0s
Test compatibility (ember-release)	57s
Test compatibility (ember-canary)	31s
	10m 38s

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