[Form] add deprecation log (#12607)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12607
| License       | MIT
| Doc PR        | 

This adds [depreciation note](http://symfony.com/doc/current/contributing/code/conventions.html#deprecations) for those `ROUND_HALFEVEN`, `ROUND_HALFUP` and `ROUND_HALFDOWN` constants.
I don't have any preferences. Let's see what the core team think about it.