[Deprecated] [ClassLoader] [Validator] Fix issues #12600 & #12682

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | 12600,12682
| License       | MIT
| Doc PR        | 
use the full classname instead, see https://github.com/symfony/symfony/search?utf8=%E2%9C%93&q=trigger_error+class+has+been+deprecated&type=Code