Bump postcss and @microsoft/sp-build-web

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [@microsoft/sp-build-web](http://aka.ms/spfx). These dependencies need to be updated together.


Updates `postcss` from 8.4.23 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.4.23...8.4.31)

Updates `@microsoft/sp-build-web` from 1.11.0 to 1.18.0

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

samples/react-content-query-online/package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "@microsoft/mgt": "^2.0.1",
-    "@microsoft/sp-build-web": "1.11.0",
+    "@microsoft/sp-build-web": "1.18.0",
     "@microsoft/sp-core-library": "1.11.0",
     "@microsoft/sp-lodash-subset": "1.11.0",
     "@microsoft/sp-office-ui-fabric-core": "1.11.0",