Update dependency postcss to v8.4.29 (mastodon#26720)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

yarn.lock

@@ -9625,9 +9625,9 @@ postcss-value-parser@^4.1.0, postcss-value-parser@^4.2.0:
  integrity sha512-1NNCs6uurfkVbeXG4S8JFT9t19m45ICnif8zWLd5oPSZ50QnwMfK+H3jv408d4jw/7Bttv5axS5IiHoLaVNHeQ==
 
 postcss@^8.2.15, postcss@^8.4.24, postcss@^8.4.25:
- version "8.4.28"
- resolved "https://registry.yarnpkg.com/postcss/-/postcss-8.4.28.tgz#c6cc681ed00109072816e1557f889ef51cf950a5"
- integrity sha512-Z7V5j0cq8oEKyejIKfpD8b4eBy9cwW2JWPk0+fB1HOAMsfHbnAXLLS+PfVWlzMSLQaWttKDt607I0XHmpE67Vw==
+ version "8.4.29"
+ resolved "https://registry.yarnpkg.com/postcss/-/postcss-8.4.29.tgz#33bc121cf3b3688d4ddef50be869b2a54185a1dd"
+ integrity sha512-cbI+jaqIeu/VGqXEarWkRCCffhjgXc0qjBtXpqJhTBohMUjUQnbBr0xqX3vEKudc4iviTewcJo5ajcec5+wdJw==
  dependencies:
  nanoid "^3.3.6"
  picocolors "^1.0.0"