Fix return type for Enum::instances()

[devster]

Hi Elao team!

We are switching from myclabs/php-enum to your package because of your great integration with symfony (mostly DBAL types and serializer).

It seems that the return type of the Enum::instances() method does not suit well for our dear friend phpstan :D

I suppose this is not done on purpose since an override of the return type has been done for the ::get() method:
EnumInterface::get(): EnumInterface -> Enum::get(): static

So I suggest to do the same with the Enum::instances() method.

Cheers, and gros poutous

[chalasr]

👍 build failures all seem unrelated

[ogizanagi]

Thanks and safe poutous ;)

[devster]

Thx!