Update PHP dependencies for composer 2.0

[phil-davis]

composer v2.0 is now out, and dg/composer-cleaner requires updating to the latest version to support that.
Update the PHP dependencies:

$ composer update
Updating dependencies
Lock file operations: 0 installs, 3 updates, 0 removals
  - Upgrading aws/aws-sdk-php (3.158.14 => 3.158.18)
  - Upgrading dg/composer-cleaner (v2.1 => v2.2)
  - Upgrading symfony/polyfill-mbstring (v1.19.0 => v1.20.0)
Writing lock file

And behat dependencies with composer 2.0 only work with PHP 7.4. Set the acceptance test runner to use PHP 7.4.

Part of issue owncloud/core#38067