Report error for result types with wildcard extends bound

Nobody has claimed this yet.

MapStruct does not support result types with an extends bound. For example
```
import java.util.List;
import org.mapstruct.Mapper;

@Mapper
public interface BoundMapper {
List toExtends(List in);
}
```
is invalide, because of the wildcard extends bound `? extends `. MapStruct reports this as

> Can't generate mapping method for a wildcard extends bound result.

Currently the plugin doesn't report this as an error.

One possible QuickFix could be removing the wildcard extends bound `? extends `.

Start by reproducing the issue with the BoundMapper example in IntelliJ and inspect how the plugin handles result types with wildcard extends bounds. Done means the plugin reports an error for this case, with removing the `? extends` bound as a possible QuickFix; the payload names no source file or test.

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