Bump symfony/event-dispatcher from 3.4.0 to 3.4.21

Bumps [symfony/event-dispatcher](https://github.com/symfony/event-dispatcher) from 3.4.0 to 3.4.21.
- [Release notes](https://github.com/symfony/event-dispatcher/releases)
- [Changelog](https://github.com/symfony/event-dispatcher/blob/master/CHANGELOG.md)
- [Commits](symfony/event-dispatcher@v3.4.0...v3.4.21)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support