PIM-2977: Replace discriminator map impl by mapped superclass

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | Y
| CI currently passes? | Y
| Tests pass?          | ?
| Scenarios pass?      | ?
| Checkstyle issues?*  | ?
| PMD issues?**        | ?
| Changelog updated?   | ?
| Fixed tickets        | PIM-2977
| Doc PR               |