EntityTypeInterface::getKey() is typehinted as possibly returning bool, but never returns true

Problem/Motivation

\Drupal\Core\Entity\EntityTypeInterface::getKey is incorrectly typed and causes static analysis tools like PHPStan to trip up.

Steps to reproduce

Run PHPStan on max where code calls \Drupal\Core\Entity\EntityTypeInterface::getKey. PHPStan will assume the returned variable is either string or bool. Where in fact there is never TRUE.

Proposed resolution

Change typehint from string|bool to string|false.

Remaining tasks

Implement