chore(deps): update dependency postcss7 to v8 #137

Job	Run time
Build and Test (ubuntu-latest, 18)	1m 30s
Build and Test (ubuntu-latest, 20)	2m 2s
Build and Test (ubuntu-latest, 22)	1m 48s
Build and Test (windows-latest, 18)	2m 53s
Build and Test (windows-latest, 20)	2m 43s
Build and Test (windows-latest, 22)	2m 54s
Build and Test (macos-latest, 18)	2m 22s
Build and Test (macos-latest, 20)	1m 57s
Build and Test (macos-latest, 22)	1m 35s
	19m 44s

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