Fix Call to an undefined method $this(Drupal\views\Plugin\views\HandlerBase)::getFormula()

Problem/Motivation

Split off from #3320569: Add an interface for operators() on views

Should this be an interface in the same way? Maybe warrants further discussion?

@longwave in MR of #3320569: Add an interface for operators() on views

If we are fixing the other issue by adding an interface, shouldn’t we do that here too?

@lendude on Slack

Steps to reproduce

Introduce an interface and deprecation notice similar to #3320569: Add an interface for operators() on views

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet