Maintain all fixed versions on CVE record after upstream git branches are deleted

[sethmlarson]

CPython deletes branches for versions that are no longer in security fix mode. This leads our automatic CVE record updater to no longer traverse the git commits to discover the fix version for a particular fix commit.

Instead of updating the records automatically in this case we should probably just freeze the record in place so they are no longer updated automatically after some time to avoid this issue.