Add note about removing Handler<AsyncResult<T>> from Future to migration guide.

[LuboViluda]

Describe the feature

• Add a note about the change to the migration guide.
• Remove misleading part: "Future extends Handler<AsyncResult> for some time now (and the default implementation actually returns this)." source, which is no longer valid as Future doesn't extend Handler<AsyncResult now.

Vert.x Future extends Handler<AsyncResult>
Vert.x 3.9

Vert.x Future doesn't extend Handler<AsyncResult> in Vert.x 4.0 Vert.x 4.0

As the handle(AsyncResult asyncResult) method is missing this is a potentially breaking change.

Use cases

• users of Vert.x are prompted about potentially breaking change.