Does not support guzzle 7.x

[tigerbook73]

When following the instruction in my project, the following error occurs when run "composer update"

Your requirements could not be resolved to an installable set of packages.

Problem 1
- digitalriver/digital-river-php dev-main requires guzzlehttp/guzzle ^6.2 -> found guzzlehttp/guzzle[6.2.0, ..., 6.5.x-dev] but it conflicts with your root composer.json require (^7.2).
- Root composer.json requires digitalriver/digital-river-php *@dev -> satisfiable by digitalriver/digital-river-php[dev-main].

Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.