Fix: Correct reference to Stripe Next Gen gateway when loading legacy Stripe code

[kjohnson]

Description

As reported by @Benunc, the Next Gen feature plugin was causing a break in form styles for the (legacy) Multi-Part template. I narrowed down the issue to the Next Gen Test Gateway and the (legacy) Stripe front-end files, which are included for the webhook listeners.

The loadLegacyStripeWebhooksAndFilters() method was checking for the NextGenTestGateway::class to be "active", but should instead have been checking for the NextGenStripeGateway::class - a simple typo in the code and oversight during code review.

Additionally, there issue revealed a bug in the legacy Stripe code, in which a callback for the filter hook give_form_html_tags returned false if no Stripe gateways were active when the filter hook expects an array to always be returned.

Visuals

Broken	Fixed

Testing Instructions

Using a (legacy) Multi-Part form template:

• Toggle the Next Gen Test Gateway
• Check the form field styles

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[JasonTheAdams]

Good find, @kjohnson! Please shoot this over to QA for a quick test.

[Benunc]

appears to work now!