[8.x] Fix Mailer contract returns

[driesvints]

All other factory contracts return an implementation of a contract. I think the original reason why this was a concrete is because the MailManager's resolve method always returns a concrete implementation of Illuminate\Mail\Mailer. This is fine and while this technically means that the MailManager will always return an instance of Illuminate\Mail\Mailer it's probably best to loosen the implementation on the Factory contract to allow any implementation of a Mailer type. The Mail component should function perfectly fine with any mailer implementing this contract type.

No breaking change because only DocBlocks were edited and existing code will continue to work as is.

[GrahamCampbell]

Is this issue in 6.x too?

[driesvints]

@GrahamCampbell not really an issue. More like a widening of the contract