[2.7] adds deprecation notices.

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

you should remove the trigger here IMO, because the default API level in Symfony 2.x is 2.5-BC, which will use this class. Instead, you should trigger warnings only in the BC layers implemented in this class (so that it triggers warnings only when using the 2.4 API).
The way to trigger warnings here is the same than in other places where we define a BC layer. The only reason why we have the different API levels here is because the class implementing both APIs at the same time is incompatible with PHP 5.3.8 and lower, and we had to support them as well.