PMProEmail class attributes created dynamically throw deprecated warn…

[MaximilianoRicoTabo]

…ings in PHP 8.2

• Declare $headers and $attachments as class attributes.

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Resolves #2881

How to test the changes in this Pull Request:

1. Switch PHP to 8.2 and enable wp_debug to true.
2. Try cancelling a membership without this patch and see the warning
3. Apply this patch and repeat steps above. Warning should have gone.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.