Populate `uuid` values for existing `security_findings` records

Why are we doing this work

We are in the process of deprecating the usage of the project_fingerprint attribute of the findings. By using uuid values to identify findings, we can easily associate any related entity with finding.

As we will already have some records in the security_findings table, we should populate the uuid values for them by re-downloading the artifacts. We already have the retention period of a week for these records but populating the values will be important for the on-premise installations of Gitlab.

Implementation plan

• backend Write a background migration to populate uuid values for the existing security_findings records