Error adding new breakpoints

Problem/Motivation

When trying to add additional breakpoints on the DFP Tag form, clicking the "Add another breakpoint" button doesn't add additional rows to the form. Checking the logs the following error shows:

TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, non-static method Drupal\dfp\Form\Tag::moreBreakpointsSubmit() cannot be called statically in call_user_func_array() (line 114 of /path/to/project/web/core/lib/Drupal/Core/Form/FormSubmitter.php).

Steps to reproduce

Add or edit a DFP Tag and click the "Add another breakpoint" button. I'm using PHP 8.1 when experiencing this.

Proposed resolution

Change moreBreakpointsSubmit() and moreBreakpointsJs() to static methods.

Patch to follow.