Bump symfony/phpunit-bridge from 2.8.28 to 2.8.50

Bumps [symfony/phpunit-bridge](https://github.com/symfony/phpunit-bridge) from 2.8.28 to 2.8.50.
- [Release notes](https://github.com/symfony/phpunit-bridge/releases)
- [Changelog](https://github.com/symfony/phpunit-bridge/blob/master/CHANGELOG.md)
- [Commits](symfony/phpunit-bridge@v2.8.28...v2.8.50)

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