[WIP] Guzzle7 for core

[phil-davis]

On top of PR #38266 - see what happens when Guzzle7 is used:

$ composer update
Loading composer repositories with package information
Updating dependencies
Lock file operations: 2 installs, 4 updates, 0 removals
  - Upgrading guzzlehttp/guzzle (6.5.5 => 7.2.0)
  - Upgrading league/flysystem (1.0.70 => 1.1.3)
  - Locking league/mime-type-detection (1.5.1)
  - Locking psr/http-client (1.0.1)
  - Upgrading symfony/service-contracts (v1.1.9 => v2.2.0)
  - Upgrading symfony/translation-contracts (v1.1.10 => v2.3.0)
Writing lock file

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[phil-davis]

Rebased

[phil-davis]

See #38707