Resolve `jsonSerialize` deprecation warnings

The following warning was presented for the `jsonSerialize()` methods on the `MultivariateFeatureStateValueModelList` and `CollectionTrait` and classes. We have temporarily resolved this in [this PR](https://github.com/Flagsmith/flagsmith-php-client/pull/34) by indicating that the return type will change in the next major version.

```
Deprecated: Return type of Flagsmith\Utils\Collections\FlagModelsList::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```

Comienza revisando PR 34 y las implementaciones de jsonSerialize() en MultivariateFeatureStateValueModelList, CollectionTrait y FlagModelsList. Confirma qué enfoque del tipo de retorno es temporal actualmente y, a continuación, ejecuta las comprobaciones relevantes del proyecto para verificar que la advertencia de deprecación ya no se emite sin romper las versiones de PHP compatibles.

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