A number of projects generate code quality reports that have severity
levels with typos (e.g. `marjor`) or with capitalized letters
(e.g. `MAJOR`). This would cause an error in our code quality parser
and cause a 204 No Content response for the controller endpoint.

We should categorize the former as `unknown` and make the severity
name case-insensitive. The frontend already handles unknown values.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/351276

Changelog: fixed