Bump symfony/polyfill-mbstring from 1.29.0 to 1.30.0

Bumps [symfony/polyfill-mbstring](https://github.com/symfony/polyfill-mbstring) from 1.29.0 to 1.30.0.
- [Commits](symfony/polyfill-mbstring@v1.29.0...v1.30.0)

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

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