Hotfix needed for Active Campaign API v1 issue

[DennisNDean]

The Active Campaign V1 API has started returning a 403 forbidden error when the API key is passed in the body of the request, but it works if it's in the header.

See: https://community.activecampaign.com/t/acces-suddenly-refuses/21272
https://community.activecampaign.com/t/activecampaign-api-v1-403-error/21273

Changing includes/service-providers/active_campaign/class-newspack-newsletters-active-campaign.php starting at line 167 from

$args     = [
	'method'  => $method,
	'headers' => [
		'Content-Type' => $content_type,
		'Accept'       => 'application/json',
	],
	'body'    => $body,
];

to

$args     = [
	'method'  => $method,
	'headers' => [
		'Content-Type' => $content_type,
		'Accept'       => 'application/json',
		'API-TOKEN'	   => $credentials['key'],
	],
	'body'    => $body,
];

fixes the issue.