Bump symfony/yaml from 4.4.19 to 4.4.21
Bump symfony/yaml from 4.4.19 to 4.4.21

Bumps [symfony/yaml](https://github.com/symfony/yaml) from 4.4.19 to 4.4.21.
- [Release notes](https://github.com/symfony/yaml/releases)
- [Changelog](https://github.com/symfony/yaml/blob/5.x/CHANGELOG.md)
- [Commits](https://github.com/symfony/yaml/compare/v4.4.19...v4.4.21)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
trellis-ldp/static-ldp,trellis-ldp/static-ldp