PHP 8.4 compatibility

No major issues found so far, other than the deprecation related to nullable types.

What remains in the current MR is to make explicitly nullable other cases where, due to being old code, type declaration was not added in the first place. For example, $sync_id = NULL should be ?string $sync_id = NULL. These are not flagged by static analysis tools and do not raise the deprecation warnings, but in many cases the type declarations should be added - where we have scalar types and we don't need union types.

Adding types in cases such as $sync_id = NULL i.e. ?string $sync_id = NULL does not cause a problem with inheriting classes/methods. However, in some cases - like in event subscribers provided by custom or contrib modules, calling for example the state manager - it can cause failures if the caller was passing a string while an integer is now required. Previously it was indicated that an integer is required in the docblocks, however it would work if passing a string as well due to lack of strict types.

Therefore we will add type declarations in such cases - together with general use of strict types and type declarations everywhere at Entity Synchronization v6.