Use $this->t() instead of t() in form

Problem/Motivation

Not best practice to use the procedural t() in OOP Code.
File: commerce_rave/src/Plugin/Commerce/PaymentGateway/Rave.php
See:
https://www.drupal.org/docs/8/api/translation-api/overview
https://api.drupal.org/api/drupal/core%21includes%21bootstrap.inc/function/t/8.6.x

Proposed resolution

Use $this->t();

Remaining tasks

1. Create patch
2. Review/RTBC
3. Commit

User interface changes

None

API changes

None

Data model changes

None