Bump symfony/routing from 3.4.20 to 3.4.21

Bumps [symfony/routing](https://github.com/symfony/routing) from 3.4.20 to 3.4.21.
- [Release notes](https://github.com/symfony/routing/releases)
- [Changelog](https://github.com/symfony/routing/blob/master/CHANGELOG.md)
- [Commits](symfony/routing@v3.4.20...v3.4.21)

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