Bumps [symfony/http-kernel](https://github.com/symfony/http-kernel) from 2.6.4 to 2.8.51. **This update includes security fixes.** - [Release notes](https://github.com/symfony/http-kernel/releases) - [Changelog](https://github.com/symfony/http-kernel/blob/master/CHANGELOG.md) - [Commits](https://github.com/symfony/http-kernel/compare/v2.6.4...v2.8.51) Signed-off-by: dependabot-preview[bot] <support@dependabot.com>