Bump symfony/flex from 1.9.4 to 1.9.6

Bumps [symfony/flex](https://github.com/symfony/flex) from 1.9.4 to 1.9.6.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v1.9.4...v1.9.6)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>