[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
We usually add spaces at the beginning of a comment

```
// Deprecated..
```