Fix GH-9186 @strict-properties can be bypassed using unserialization (#9354)

* Emit deprecation warnings when adding dynamic properties to classes during unserialization - this will become a

Fix GH-9186 @strict-properties can be bypassed using unserialization (#9354)

* Emit deprecation warnings when adding dynamic properties to classes during unserialization - this will become an Error in php 9.0.
(Adding dynamic properties in other contexts was already a deprecation warning - the use case of unserialization was overlooked)
* Throw an error when attempting to add a dynamic property to a `readonly` class when unserializing
* Add new serialization methods `__serialize`/`__unserialize` for SplFixedArray to avoid creating deprecated dynamic
properties that would then be added to the backing fixed-size array
* Don't add named dynamic/declared properties (e.g. $obj->foo) of SplFixedArray to the backing array when unserializing
* Update tests to declare properties or to expect the deprecation warning
* Add news entry

Co-authored-by: Tyson Andre <tysonandre775@hotmail.com>

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