Bump symfony/flex from 2.8.2 to 2.9.0 (#358)

Bumps [symfony/flex](https://github.com/symfony/flex) from 2.8.2 to 2.9.0.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v2.8.2...v2.9.0)

---
updated-dependencies:
- dependency-name: symfony/flex
  dependency-version: 2.9.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]