Resolve jsonSerialize deprecation warnings #36
Description
The following warning was presented for the jsonSerialize() methods on the MultivariateFeatureStateValueModelList and CollectionTrait and classes. We have temporarily resolved this in this PR 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