Remove CVSSv2 scores from vulnerablecode

[TG1999]

Reference: #889 (comment)
It will be a 3 step process:

• Mark all advisories with CVSSv2 with a flag so improvers don't process them in the future.
• Check none of our current importers import cvsssv2 score
• Writing a migration to remove all CVSSv2 score from the severity table.

[pombredanne]

IMHO we have this alternative:

1. delete everything about CVSSv2 including advisory and check if there are data sources that provide only CVSSv2 and how we can convert CVSSv2 into CVSSv3
2. or carry some CVSSv2 in advisories and have flags to avoid reprocessing and have some more code for CVSSv2 here and there

[pombredanne]

See also https://security.stackexchange.com/questions/127335/how-to-convert-risk-scores-cvssv1-cvssv2-cvssv3-owasp-risk-severity

[ziadhany]

I think I got a really interesting result, take a look at https://www.kaggle.com/code/ziadhany/decision-trees-for-converting-cvss-2-to-3

[TG1999]

@pombredanne please have a look on this one!