[4.2] Allow Custom Column Name When Adding or Removing Soft Deletes

Advantages:

- When creating, the developer doesn’t have to remember that the column needs to be a nullable timestamp.
- When removing, keeps the migration file clear by still communicating the intent of the column deletion (i.e., removal of soft deletes) instead of having to use a vague `dropColumn()` that says nothing about the column’s special role.