Augment Severities produced by SAST analyzers to ensure coverage
As raised with https://gitlab.com/gitlab-org/gitlab/-/issues/6730 we should look into the Severities mapped by our ~"Category:SAST" analyzers and re-evaluate our mappings to better normalize our results.
## Actions
1. Audit severities as produced by analyzers
- [x] [`brakeman`](https://gitlab.com/groups/gitlab-org/-/epics/4004#note_415218268) (https://gitlab.com/gitlab-org/security-products/analyzers/brakeman/-/merge_requests/49)
- [x] [`eslint`](https://gitlab.com/groups/gitlab-org/-/epics/4004#note_420666214) (https://gitlab.com/gitlab-org/security-products/analyzers/eslint/-/merge_requests/68)
- [x] [`flawfinder`](https://gitlab.com/groups/gitlab-org/-/epics/4004#note_399075558) (https://gitlab.com/gitlab-org/security-products/analyzers/flawfinder/-/merge_requests/32)
- [x] [`nodejs-scan`](https://gitlab.com/groups/gitlab-org/-/epics/4004#note_399586607) (https://gitlab.com/gitlab-org/gitlab/-/issues/220847)
- [x] [`security-code-scan`](https://gitlab.com/groups/gitlab-org/-/epics/4004#note_401915784) | https://gitlab.com/gitlab-org/security-products/analyzers/security-code-scan/-/merge_requests/104
- [ ] [`sobelow`](https://gitlab.com/groups/gitlab-org/-/epics/4004#note_420669449)
1. Add missing severities for those analyzers that do not map one (currently defaulting to `Unknown`)
1. Reconsider severity mapping to normalize; i.e. evaluate against CVSS
epic