Bump guzzlehttp/guzzle from 7.12.1 to 7.15.1 in /lib/civicrm-omnipay

Bumps [guzzlehttp/guzzle](https://github.com/guzzle/guzzle) from 7.12.1 to 7.15.1.
- [Release notes](https://github.com/guzzle/guzzle/releases)
- [Changelog](https://github.com/guzzle/guzzle/blob/8.0/CHANGELOG.md)
- [Commits](guzzle/guzzle@7.12.1...7.15.1)

---
updated-dependencies:
- dependency-name: guzzlehttp/guzzle
  dependency-version: 7.15.1
  dependency-type: direct:production
...

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Author: dependabot[bot]