Bump react-router and react-router-dom in /polling-app-client

Removes [react-router](https://github.com/remix-run/react-router/tree/HEAD/packages/react-router). It's no longer used after updating ancestor dependency [react-router-dom](https://github.com/remix-run/react-router/tree/HEAD/packages/react-router-dom). These dependencies need to be updated together.


Removes `react-router`

Updates `react-router-dom` from 5.1.2 to 7.5.2
- [Release notes](https://github.com/remix-run/react-router/releases)
- [Changelog](https://github.com/remix-run/react-router/blob/main/packages/react-router-dom/CHANGELOG.md)
- [Commits](https://github.com/remix-run/react-router/commits/react-router-dom@7.5.2/packages/react-router-dom)

---
updated-dependencies:
- dependency-name: react-router
  dependency-version: 
  dependency-type: indirect
- dependency-name: react-router-dom
  dependency-version: 7.5.2
  dependency-type: direct:production
...

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Author: dependabot[bot]