Prepare for php 8.4's deprecation of implicit nullable types

[mhsdesign]

with php 8.4 implicit nullable types are deprecated: https://stitcher.io/blog/new-in-php-84#implicit-nullable-types-deprecation

These are a few usages that i found just in the interfaces:

• flow-development-collection/Neos.Cache/Classes/Frontend/FrontendInterface.php

  Line 59 in ea98fd3

  public function set(string $entryIdentifier, $data, array $tags = [], int $lifetime = null);

• flow-development-collection/Neos.Flow/Classes/Property/TypeConverterInterface.php

  Line 113 in cf6165b

  public function convertFrom($source, $targetType, array $convertedChildProperties = [], PropertyMappingConfigurationInterface $configuration = null);

... and also in neos - https://github.com/neos/neos-development-collection/blob/ee36cc29460aa2005a22b577080a85ba505a6d1f/Neos.Neos/Classes/Service/DataSource/DataSourceInterface.php#L44

As i tested it should be non breaking to adjust PropertyMappingConfigurationInterface $configuration = null to be PropertyMappingConfigurationInterface|null $configuration = null event if someone implemented it and doesnt use |null yet. That seems to be possible because the implementation narrows the type which seams legal in php. Meaning we can actually adjust that at any time so Flow 9.0 would be php8.4 compatible in that aspect at some point.