Bump postcss from 8.4.24 to 8.4.31 (#3917)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.24 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.24...8.4.31)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:production
...

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package.json

@@ -298,7 +298,7 @@
     "mv": "^2.1.1",
     "node-polyfill-webpack-plugin": "1.1.4",
     "pi-ui": "https://github.com/decred/pi-ui#4daf214b901aa9f50547cf5645594cb76e7c9e51",
-    "postcss": "8.4.24",
+    "postcss": "8.4.31",
     "postcss-loader": "^7.0.1",
     "postcss-preset-env": "7.8.3",
     "prop-types": "15.8.1",

yarn.lock

@@ -10850,10 +10850,10 @@ postcss-values-parser@^2.0.1:
     indexes-of "^1.0.1"
     uniq "^1.0.1"
 
-postcss@8.4.24, postcss@^8.1.7, postcss@^8.2.15:
-  version "8.4.24"
-  resolved "https://registry.yarnpkg.com/postcss/-/postcss-8.4.24.tgz#f714dba9b2284be3cc07dbd2fc57ee4dc972d2df"
-  integrity sha512-M0RzbcI0sO/XJNucsGjvWU9ERWxb/ytp1w6dKtxTKgixdtQDq4rmx/g8W1hnaheq9jgwL/oyEdH5Bc4WwJKMqg==
+postcss@8.4.31, postcss@^8.1.7, postcss@^8.2.15:
+  version "8.4.31"
+  resolved "https://registry.yarnpkg.com/postcss/-/postcss-8.4.31.tgz#92b451050a9f914da6755af352bdc0192508656d"
+  integrity sha512-PS08Iboia9mts/2ygV3eLpY5ghnUcfLV/EXTOW1E2qYxJKGGBUtNjN76FYHnMs36RmARn41bC0AZmn+rR0OVpQ==
   dependencies:
     nanoid "^3.3.6"
     picocolors "^1.0.0"