Remove `NOT NULL` constraint for `security_findings` on `project_fingerprint` column

Why are we doing this work

We are in the process of deprecating the usage of the project_fingerprint attribute of the findings. As there is a null constraint associated to project_fingerprint, it will be difficult to remove the use of project_fingerprint from code without removing this constraint.

Implementation plan

• backend Create a migration to remove NOT NULL constraint on project_fingerprint column of the security_findings table