Bump behat/mink from 1.7.1 to 1.8.1

Bumps [behat/mink](https://github.com/minkphp/Mink) from 1.7.1 to 1.8.1.
- [Release notes](https://github.com/minkphp/Mink/releases)
- [Changelog](https://github.com/minkphp/Mink/blob/master/CHANGES.md)
- [Commits](minkphp/Mink@v1.7.1...v1.8.1)

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Author: dependabot-preview[bot]