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        | 

This is a bad idea, because it means that Symfony will trigger deprecation warnings when instantiating the 2.5-BC API (which is the default to preserve BC), even if your code uses the new API methods