Bump symfony/polyfill-mbstring from 1.28.0 to 1.29.0

Bumps [symfony/polyfill-mbstring](https://github.com/symfony/polyfill-mbstring) from 1.28.0 to 1.29.0.
- [Commits](symfony/polyfill-mbstring@v1.28.0...v1.29.0)

---
updated-dependencies:
- dependency-name: symfony/polyfill-mbstring
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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