Remove the dependency on the Entity Reference module

Problem/Motivation

Payment Reference depends on core's Entity Reference module, which was deprecated some weeks ago. All its functionality was moved to \Drupal\Core, so the dependency is redundant and we can safely remove it without introducing regressions.

Proposed resolution

Remove the explicit dependency on the Entity Reference module, and use the new classes in \Drupal\Core instead of the now deprecated ones in the Entity Reference module.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.