[Validator] Added a deprecation note about setPropertyAccessor #12698

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12698
| License       | MIT
| Doc PR        | 
`in future` is probably not needed.