Issue #3308872 by Spokje, xjm, longwave, bnjmnm: Address "postcss.plugin was deprecated" warning

core/package.json

core/scripts/css/compile.js

core/scripts/css/remove-unwanted-comments.js

core/themes/olivero/css/base/base.css

core/themes/olivero/css/base/fonts.css

core/themes/olivero/css/base/variables.css

core/themes/olivero/css/components/action-links.css

core/themes/olivero/css/components/ajax-progress.module.css

...es/olivero/css/components/autocomplete-loading.module.css

core/themes/olivero/css/components/block.css

core/themes/olivero/css/components/book.css

core/themes/olivero/css/components/breadcrumb.css

core/themes/olivero/css/components/button.css

core/themes/olivero/css/components/comments.css

core/themes/olivero/css/components/content-moderation.css

core/themes/olivero/css/components/details.css

core/themes/olivero/css/components/embedded-media.css

core/themes/olivero/css/components/feed.css

core/themes/olivero/css/components/field.css

core/themes/olivero/css/components/fieldset.css