Bump postcss from 8.4.23 to 8.4.31 in /web/signed-rsvp

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: direct:production
...

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

web/signed-rsvp/package.json

@@ -23,7 +23,7 @@
         "framer-motion": "^10.11.5",
         "headlessui": "^0.0.0",
         "next": "^13.4.2",
-        "postcss": "^8.4.21",
+        "postcss": "^8.4.31",
         "react": "^18.2.0",
         "react-dom": "^18.2.0",
         "react-icons": "^4.8.0",