[4.2] Make Soft Delete Trait Backward Compatible

4.0 and 4.1 allowed the column for soft deletes to be set to something other than the default "deleted_at", just as is possible for `created_at` and `deleted_at`. 4.2 breaks backward compatibility by hardcoding in the string "deleted_at" in `SoftDeletingTrait::getDeletedAtColumn()`.

This commit checks whether the old `DELETED_AT` constant is set before resorting to the default.