[Form] Deprecated read_only option

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | read_only in form options
| Tests pass?   | no
| Fixed tickets | #10658 
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/3824

- [ ] figure out why tests aren't passing