[6.x] Fix auto creating model by class name

[vv12131415]

I'm targeting 6.x because it was broken in 6.x by #30653

There was an issue about this problem #31180

as @driesvints said

We only support Eloquent models for this rule. Because you're not using an Eloquent model, this rule won't work. You'll have to ask the maintainers of laravel doctrine to provide a custom database rule.

But the thing is illuminate/database is only suggestion, not the requirement of this library. Another thing is that https://github.com/laravel-doctrine/ implements PresenceVerifierInterface which works perfectly fine.

The change that I've proposed does not break anyone else code, it just makes validation rule more general purpose and adds ability for other database ORM providers (the guaranties was given by PresenceVerifierInterface, I can implement it on my own and make FilePresenceVerifier if my persistence layer is in file, same way I can make ExternalApiPresenceVerifier and check external API for existence or uniqueness)

also saying that

We only support Eloquent models for this rule. Because you're not using an Eloquent model, this rule won't work. You'll have to ask the maintainers of laravel doctrine to provide a custom database rule.

Is not correct, since? as I have described you give PresenceVerifierInterface

[vv12131415]

I've force pushed to retrigger failed CI (some why container of MySQL on php 7.2 failed)

[vv12131415]

okay, reverting back to is_subclass_of
since "Illuminate\Tests\Validation\User" instanceof Model fails, instanceof only works on object level I suppose.