Replace deprecated usage of entity_create with a direct call to the entity type class in User module

Follow-up to #2490966: [Meta] Replace deprecated usage of entity_create with a direct call to the entity type class

Problem/Motivation

According to #2346261: Deprecate entity_create() in favor of a <EntityType>::create($values) or \Drupal::entityManager()->getStorage($entity_type)->create($values), entity_create() function is going to be deprecated in favor of the create() method of the entity type class if known, or the EntityManager::create() helper.
That deprecated functions is used in a lot of different contextes.

Beta phase evaluation

Proposed resolution

Replace the deprecated call to entity_create() by a proper call to <EntityType>::create() or EntityManager::create(), according to the case, everywhere its needed.

Before:

entity_create('field_config', $field_values)->save();

$type = 'node';
entity_create($type, $node_values)->save();

After:

use Drupal\field\Entity\FieldConfig;
FieldConfig::create($field_values)->save();

use Drupal\Core\Entity\EntityManager;
$type = 'node';
EntityManager::create($type, $node_values)->save();

Remaining tasks

User interface changes

None.

API changes

None.