Create IssueLink for Vulnerabilities that do not have them

Part of changes required for #224457 (closed).

Some Vulnerability objects in GitLab.com database do not have an associated Vulnerabilities::IssueLink entries. This is because of our migration to standalone Vulnerability objects. Some of those Vulnerabilities have associated Vulnerabilities::Finding objects which, in turn, have an Vulnerability::Feedback association with type of issue. We want to create IssueLinks for those issues so our customers retain access to issues created that way.

Implementation plan

• database Use EachBatch on Vulnerabilities::Feedback in a post-deployment migration to create required entries