Fix PHP 8.5 + symfony deprecations

[W0rma]

Q	A
Bug fix?	no
New feature?	no
Doc updated	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets
License	MIT

Fixes the following deprecation warnings:

• The return value of method DateTimeImmutable::setTimezone() should either be used or intentionally ignored by casting it as (void), as DateTimeImmutable::setTimezone() does not modify the object itself in /home/runner/work/serializer/serializer/tests/Serializer/BaseSerializationTestCase.php:854
• Method SplObjectStorage::attach() is deprecated since 8.5, use method SplObjectStorage::offsetSet() instead in /home/runner/work/serializer/serializer/src/SerializationContext.php:71
• Method SplObjectStorage::contains() is deprecated since 8.5, use method SplObjectStorage::offsetExists() instead in /home/runner/work/serializer/serializer/src/SerializationContext.php:101
• Method SplObjectStorage::detach() is deprecated since 8.5, use method SplObjectStorage::offsetUnset() instead in /home/runner/work/serializer/serializer/src/SerializationContext.php:84
• Since symfony/validator 7.3: Passing an array of options to configure the "Symfony\Component\Validator\Constraints\Length" constraint is deprecated, use named arguments instead. in /home/runner/work/serializer/serializer/vendor/symfony/deprecation-contracts/function.php:25
• Since symfony/validator 7.4: Support for evaluating options in the base Constraint class is deprecated. Initialize properties in the constructor of Symfony\Component\Validator\Constraints\Length instead. in /home/runner/work/serializer/serializer/vendor/symfony/deprecation-contracts/function.php:25