Problem/Motivation
From #3044175-12: PHP7.1 vendor min Testing issue - DO NOT COMMIT:
guzzlehttp/guzzle needs to be at least at 6.3 where CookieJar::getCookieByName was introduced
Proposed resolution
require guzzlehttp/guzzle ^6.3
Remaining tasks
User interface changes
API changes
Data model changes
Release notes snippet
Drupal core now explicitly requires Guzzle 6.3 or higher for the end of PHP 7.0's official security support and for forward compatibility Drupal 9's increased PHP version requirements.