[WIP] Guzzle7 for core

[phil-davis]

On top of PR #38706 - see what happens when Guzzle7 is used:

$ composer update
Loading composer repositories with package information
Updating dependencies
Lock file operations: 1 install, 1 update, 3 removals
  - Removing symfony/polyfill-intl-idn (v1.22.1)
  - Removing symfony/polyfill-intl-normalizer (v1.22.1)
  - Removing symfony/polyfill-php72 (v1.22.1)
  - Upgrading guzzlehttp/guzzle (6.5.5 => 7.3.0)
  - Locking psr/http-client (1.0.1)
Writing lock file

(This replace old PR #38267 )

[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]

Federated acceptance tests are failing.

There are also some unit test failures.

Needs investigating when someone wants to move forward with this.

[phil-davis]

This is here as a "record" of past code for updating Guzzle from v5 to v7. It should provide a good starting point if someone does this some day.

[JammingBen]

Superseded by #39368.