Bump symfony/flex from 1.20.2 to 1.21.2 (#272)

Bumps [symfony/flex](https://github.com/symfony/flex) from 1.20.2 to 1.21.2.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v1.20.2...v1.21.2)

---
updated-dependencies:
- dependency-name: symfony/flex
  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>