Update how we map Flawfinder's severity

Description

quoting gitlab-org/security-products/analyzers/flawfinder!1 (comment 72235757)

I know this is following the same behavior as previous implementation but I was wondering about this mapping to High while we also have Critical available... If there is no clear answer let's open an issue to discuss that.

Proposal

Improve severity mapping to use all possible values provided by the common library