feature #24631 [Form] Fix FormEvents::* constant and value matching (yceruto, javiereguiluz)

This PR was merged into the 4.0-dev branch.

Discussion
----------

[Form] Fix FormEvents::* constant and value matching

| Q             | A
| ------------- | ---
| Branch?       | 4.0
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes (ppl rely on const value directly, very weird)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony/symfony#24615
| License       | MIT
| Doc PR        | -

> symfony/symfony#24615 (comment) by @stof:
Yeah, I think we could change this in 4.0 without a big impact (btw, I think our BC policy even allows it for this case).
There is one case where people will use the event name rather than the constant: the kernel.event_listener tag (and recent versions can even use the constant in YAML files). But this won't be the case for this event, as form events are not dispatched in the main dispatcher anyway.

Commits
-------

944931af63 Minor reword
0ee856add1 Update UPGRADE-4.0.md
0fc2282fc2 fix the constant value to be consistent with the name

Author: fabpot

FormEvents.php

@@ -31,7 +31,7 @@ final class FormEvents
      *
      * @Event("Symfony\Component\Form\FormEvent")
      */
-    const PRE_SUBMIT = 'form.pre_bind';
+    const PRE_SUBMIT = 'form.pre_submit';
 
     /**
      * The SUBMIT event is dispatched just before the Form::submit() method
@@ -41,7 +41,7 @@ final class FormEvents
      *
      * @Event("Symfony\Component\Form\FormEvent")
      */
-    const SUBMIT = 'form.bind';
+    const SUBMIT = 'form.submit';
 
     /**
      * The FormEvents::POST_SUBMIT event is dispatched after the Form::submit()
@@ -51,7 +51,7 @@ final class FormEvents
      *
      * @Event("Symfony\Component\Form\FormEvent")
      */
-    const POST_SUBMIT = 'form.post_bind';
+    const POST_SUBMIT = 'form.post_submit';
 
     /**
      * The FormEvents::PRE_SET_DATA event is dispatched at the beginning of the Form::setData() method.