chore: bump postcss from 8.5.6 to 8.5.15 #1555

Job	Run time
Quickstart addon	1m 19s
Quickstart self-hosted model	3m 28s
Cookbook notebooks	3m 21s
Quickstart model	1m 11s
Quickstart application	1m 3s
build	2m 3s
	12m 25s

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