Bump symfony/monolog-bundle from 3.3.1 to 3.4.0

Bumps [symfony/monolog-bundle](https://github.com/symfony/monolog-bundle) from 3.3.1 to 3.4.0.
- [Release notes](https://github.com/symfony/monolog-bundle/releases)
- [Changelog](https://github.com/symfony/monolog-bundle/blob/master/CHANGELOG.md)
- [Commits](symfony/monolog-bundle@v3.3.1...v3.4.0)

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