Bump symfony/flex from 1.21.5 to 1.21.6

Bumps [symfony/flex](https://github.com/symfony/flex) from 1.21.5 to 1.21.6.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v1.21.5...v1.21.6)

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

Signed-off-by: dependabot[bot] <[email protected]>