Revert replace default_value_for on ApprovalProjectRule model

What does this MR do and why?

Related to #381742 (closed)

It turns out that we have a lot of records in the database(203024 to be precise) that have this value set to NULL. Because of these changes updates to those records will fail.

It looks like we do have 203024 records for which the scanners attribute is NULL in the database.

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