Add model-level default for Vulnerability#detected_at
What does this MR do and why?
This prevents BulkInsertSafe from setting this attribute to nil. This change needs to be followed by a batched background migration to backfill the values.
It appears that our BulkInsertSafe will insert NULL to every value which is not set which prevents Rails from using DEFAULT when submitting the query to be executed which happily discarded the default value set on the database-level. See !179644 (comment 2330916283) for more details
This is the simplest approach to fix that particular problem. I believe that BulkInsertSafe is not behaving correctly but this is a separate matter for now.