Disallow NULL values in detected_at colum

Why are we doing this work?

After #336089 (closed) gets merged we need to disallow NULL values in the detected_at column

Implementation plan

database Create a post-deployment migration according to step 1 of Add a NOT NULL constraint to an existing column
schedule #336092 (closed) for execution in the subsequent milestone

Edited Jul 15, 2021 by Thiago Figueiró