build: Updating postcss to 8.2.15
build: Updating postcss to 8.2.15

* https://npmjs.com/advisories/1693 (CVE-2021-23368)

Change-Id: I76c543ff8ed52814f4ec0620a87967470af72bd2

wikimedia/mediawiki-extensions-GoogleAdSense,wikimedia/mediawiki-extensions-GoogleAdSense