Deprecate `setFromDb()` method

[promatik]

Related with https://github.com/Laravel-Backpack/docs/pull/506.

[pxpm]

I agree we do this, but at the same time we remove setFromDB from our stubs too.

Otherwise I don't think it makes sense to deprecate a function that we still use.

The stubs are now publishable too, so if you want to add something back that we removed you can just publish the stubs and do your changes accordingly.

Cheers