Bump guzzlehttp/guzzle from 6.5.5 to 6.5.6 (ampproject#7119)

Bumps [guzzlehttp/guzzle](https://github.com/guzzle/guzzle) from 6.5.5 to 6.5.6.
- [Release notes](https://github.com/guzzle/guzzle/releases)
- [Changelog](https://github.com/guzzle/guzzle/blob/6.5.6/CHANGELOG.md)
- [Commits](guzzle/guzzle@6.5.5...6.5.6)

---
updated-dependencies:
- dependency-name: guzzlehttp/guzzle
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>