[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.
This won't work. The deprecated constants have the same values than their replacements, so you cannot know whether the deprecated name or the uptodate name were used when passing the option. You will always trigger the deprecation warning here