Bump postcss from 8.4.23 to 8.4.31 in /ui-tests (#7088)

Bumps [postcss](https://github.com/postcss/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](postcss/postcss@8.4.23...8.4.31)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

ui-tests/yarn.lock

@@ -3134,13 +3134,13 @@ __metadata:
   linkType: hard
 
 "postcss@npm:^8.3.11":
-  version: 8.4.23
-  resolution: "postcss@npm:8.4.23"
+  version: 8.4.31
+  resolution: "postcss@npm:8.4.31"
   dependencies:
     nanoid: ^3.3.6
     picocolors: ^1.0.0
     source-map-js: ^1.0.2
-  checksum: 8bb9d1b2ea6e694f8987d4f18c94617971b2b8d141602725fedcc2222fdc413b776a6e1b969a25d627d7b2681ca5aabb56f59e727ef94072e1b6ac8412105a2f
+  checksum: 1d8611341b073143ad90486fcdfeab49edd243377b1f51834dc4f6d028e82ce5190e4f11bb2633276864503654fb7cab28e67abdc0fbf9d1f88cad4a0ff0beea
   languageName: node
   linkType: hard