[Hackday] [Form] Added deprecation notice on Form::getErrorsAsString

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12611
| License       | MIT
| Doc PR        | -
please replace "has been deprecated" by "is deprecated since 2.X and will be removed in 3.0"