add deprecation log (#12674)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12674
| License       | MIT
| Doc PR        | 

@weaverryan no. they might not rely on the old API (it depends how they call the validator). They might just be using the default Symfony configuration (i.e. with BC layers enabled).

I doubt anyone will write their own implementation of this interface (which is where triggering this warning on autoload would give a useful message).
and for anyone typehinting the interface, this deprecation warning is useless, given that typehints don't trigger autoloading. This would be triggered when instantiating a class to fulfill the typehint, but our BC class affects all Symfony users