[WIP] Guzzle6 for core

[phil-davis]

Description

• update dependency for Guzzle to 6.5
• get adjusted code from old PR [WIP] Guzzle6 for core #37011 (that itself came from PR Acceptance Tests for case-sensitive file names #32357 )
• do whatever is needed to make it work
• enable Tests/TestHelpers/Unit tests again (they rely on having guzzle6 since the acceptance test code was updated to guzzle6)
• Update guzzle and other dependencies in apps/files_external/3rdparty like 6225510

Related Issue

Motivation and Context

Guzzle5 does not have official support on PHP 7.4 and probably never will have. Guzzle5 works on PHP 7.4
For future PHP versions (e.g. PHP 8.0) we might be forced to do this Guzzle update.

Note1: if we do this then apps that use Guzzzle5 will also need updating to Guzzle6 and coordinated release of core and app versions etc.

Note2: guzzle is actually up to release https://github.com/guzzle/guzzle/releases/tag/7.2.0 - so if we bump guzzle we might want to bump it all the way to major version 7.

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[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 PR #38706