PIM-3255 Remove the 'defaultValue' property of Attribute

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | Y
| CI currently passes? | N
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3255
| DB schema updated?   | Y
| Migration script?    | Y
| Doc PR               | ~
Is there no way to get the object manager from somewhere.
This script won't work for projects which override attribute entity.