Bump symfony/flex from 1.9.3 to 1.9.4
Bump symfony/flex from 1.9.3 to 1.9.4

Bumps [symfony/flex](https://github.com/symfony/flex) from 1.9.3 to 1.9.4.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](https://github.com/symfony/flex/compare/v1.9.3...v1.9.4)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
sonata-project/sandbox,sonata-project/sandbox,sonata-project/sandbox