Fix Composer 2 compatibility

[brandonkelly]

The ryanwinchester/hubspot-php packages that this plugin requires has been marked as abandoned, and is now available as hubspot/hubspot-php.

The plugin requires v1 of the library, which is getting a deprecation notice when installing from Composer 1:

Deprecation Notice: Class SevenShores\Hubspot\Exceptions\HubspotException located in ./vendor/ryanwinchester/hubspot-php/src/Exceptions/HubSpotException.php does not comply with psr-4 autoloading standard. It will not autoload anymore in Composer v2.0.

Composer 2 was released on October 24, and is now the default version that will be installed, so it’s critical that this gets resolved ASAP, or people will start getting Class not found errors.

The error appears to have been resolved in v2, and also appears to have minimal breaking changes from v1.

(Note that v3 of the library will not currently work with Craft, as it requires Guzzle 7 and PHP 7.2+.)