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        | 

Great catch @stof. Could we update the message? If they are using the 2.5-BC API, then they are still kind of relying on the deprecated API (even if they are not actually using any of its old functionality anywhere). We could say something like:

>
ValidatorInterface was deprecated in version 2.5 and will be removed in version 3.0. Please use Validator\\ValidatorInterface instead and make sure you're not relying on the 2.5-BC API in any way.

Better? If we revert, I can't think of another way to have this warning.