Bump symfony/yaml from 6.4.20 to 6.4.21 in /web/landing

Bumps [symfony/yaml](https://github.com/symfony/yaml) from 6.4.20 to 6.4.21.
- [Release notes](https://github.com/symfony/yaml/releases)
- [Changelog](https://github.com/symfony/yaml/blob/7.2/CHANGELOG.md)
- [Commits](symfony/yaml@v6.4.20...v6.4.21)

---
updated-dependencies:
- dependency-name: symfony/yaml
  dependency-version: 6.4.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]