Fix PHP 8.4 implicit nullable parameter deprecation in ContentEntity::create()

Problem/Motivation

On PHP 8.4, `ContentEntity::create()` triggers a deprecation warning:

> Implicitly marking parameter $migration as nullable is deprecated, the explicit nullable type must be used instead

This occurs at `src/Plugin/migrate/source/d8/ContentEntity.php` line 62.

Steps to reproduce

1. Run any migration using the `d8_entity` source plugin on PHP 8.4.
2. Observe the deprecation warning in the output.

Proposed resolution

Add the explicit nullable type indicator (`?`) to the `$migration` parameter:

// Before
public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {

// After
public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, ?MigrationInterface $migration = NULL) {

This is the only instance in the module. The deprecated subclass plugins (d8_node, d8_user, d8_file, d8_taxonomy_term) inherit from ContentEntity and do not override create().

Remaining tasks

Review and merge.

User interface changes

API changes

Data model changes