Bump guzzlehttp/guzzle from 6.3.0 to 6.5.8 in /user.bak/plugins/admin

Bumps [guzzlehttp/guzzle](https://github.com/guzzle/guzzle) from 6.3.0 to 6.5.8.
- [Release notes](https://github.com/guzzle/guzzle/releases)
- [Changelog](https://github.com/guzzle/guzzle/blob/6.5.8/CHANGELOG.md)
- [Commits](guzzle/guzzle@6.3.0...6.5.8)

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

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