Bump postcss and @microsoft/sp-build-web in /samples/react-image-gallery

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 6.0.1 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/6.0.1...8.4.31)

Updates `@microsoft/sp-build-web` from 1.14.0 to 1.18.0

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

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

samples/react-image-gallery/package.json

@@ -23,7 +23,7 @@
   "devDependencies": {
     "@types/react": "16.9.51",
     "@types/react-dom": "16.9.8",
-    "@microsoft/sp-build-web": "1.14.0",
+    "@microsoft/sp-build-web": "1.18.0",
     "@microsoft/sp-tslint-rules": "1.14.0",
     "@microsoft/sp-module-interfaces": "1.14.0",
     "@microsoft/rush-stack-compiler-3.9": "0.4.47",