Bump symfony/flex from 1.11.0 to 1.12.1 in /symfony

Bumps [symfony/flex](https://github.com/symfony/flex) from 1.11.0 to 1.12.1.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v1.11.0...v1.12.1)

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

Author: dependabot[bot]