minor: deprecate auto-persist configuration

[nikophil]

No description provided.

[kbond]

To clarify, the way to achieve is to use ->withoutPersisting() on your factory?

[nikophil]

@kbond yep, there are multiple ways to do this:

• indeed, use ->withoutPersisting() on your factory
• use the helper disable_persisting() which disables it globally, for the current test

[nikophil]

by the way, this was not tested, nor documented and I've never seen it used in the wild. It is also a small annoyance, because before accessing PersistentObjectFactory::$persistMode you have to check if it is actually initialized.

For all these reasons, I think the best is to deprecate it. But I'm fine to keep it if you think we should ☺️

[kbond]

Let's deprecate and see if we hear anything