minor symfony#18772 Update UPGRADE FROM 2.x to 3.0 (bradbyu)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes symfony#18772).

Discussion
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Update UPGRADE FROM 2.x to 3.0

| Q             | A
| ------------- | ---
| Branch?       | "master"
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | deprecated in 2.7 removed in 3.0
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | http://symfony.com/doc/2.8/reference/forms/types/text.html for starters

You missed the Form max_length option deprecation in several field types (Text, Password, Email, etc...).

Commits
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cd66a45 Update UPGRADE FROM 2.x to 3.0

Author: nicolas-grekas

UPGRADE-3.0.md

@@ -132,7 +132,10 @@ UPGRADE FROM 2.x to 3.0
 
 ### Form
 
- *  The `ChoiceToBooleanArrayTransformer`, `ChoicesToBooleanArrayTransformer`,
+ * The `max_length` option was removed. Use the `attr` option instead by setting it to
+   an `array` with a `maxlength` key.
+
+ * The `ChoiceToBooleanArrayTransformer`, `ChoicesToBooleanArrayTransformer`,
     `FixRadioInputListener`, and `FixCheckboxInputListener` classes were removed.
 
  * The `choice_list` option of `ChoiceType` was removed.