Change BlazyFileFormatterBase::getEntitiesToView based on core patch

Problem/Motivation

Applying the Drupal Core patch in #3138528: getEntitiesToView() should implements the same interface as viewElements() → causes the site to throw PHP warnings:

Warning: Declaration of Drupal\blazy\Plugin\Field\FieldFormatter\BlazyFileFormatterBase::getEntitiesToView(Drupal\Core\Field\EntityReferenceFieldItemListInterface $items, $langcode) should be compatible with Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase::getEntitiesToView(Drupal\Core\Field\FieldItemListInterface $items, $langcode)

Steps to reproduce

- Apply the above patch
- Visit a page where Blazy might be called.
- Observe the error

Proposed resolution

Patch Blazy accordingly.

Remaining tasks

This won't be mergable until the related task is.