Security reports: Remodel severity levels
### Proposal
Rework severity levels to match [CVSS v3](https://www.first.org/cvss/calculator/3.0)
* 1 issue to update the mapping to remove `Undefined` and use `Unknown` instead in our analyzers https://gitlab.com/gitlab-org/gitlab/issues/198157
* 1 issue to update the rails backend to remove the `Undefined` value and migrate existing records in DB having `Undefined` to `Unknown`. https://gitlab.com/gitlab-org/gitlab/issues/198158
* 1 issue to cleanup rails ~frontend to remove the handling of `Undefined` value https://gitlab.com/gitlab-org/gitlab/issues/198159
* 1 issue to update existing documentation about possible values for Severity to remove `Undefined` and document the mapping for each analyzer https://gitlab.com/gitlab-org/gitlab/issues/198160
* 1 Clearly explain `Unknown` means `no severity data` within the UI. https://gitlab.com/gitlab-org/gitlab/issues/198161
* 1 Clearly explain `Unknown` means `no severity data` within the docs. https://gitlab.com/gitlab-org/gitlab/issues/198162
* 1 Document how we normalize our severity values into our framework for each analyzer https://gitlab.com/gitlab-org/gitlab/issues/198163
epic