Bump symfony/flex from 2.7.1 to 2.8.1

Bumps [symfony/flex](https://github.com/symfony/flex) from 2.7.1 to 2.8.1.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v2.7.1...v2.8.1)

---
updated-dependencies:
- dependency-name: symfony/flex
  dependency-version: 2.8.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]