[Form] Fix #10658 Deprecate "read_only" option

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #10658
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/3782
You need to reverse the logic. According to html spec a boolean property is active when it is set (whatever the value). So `readonly="0"` is also readonly.
So I'd suggest `$readOnly =  isset($view->vars['attr']['readonly']) && false !== $view->vars['attr']['readonly']`