Handle non-standard severities better in codequality degradations

What does this MR do and why?

This merge request updates CodequalityDegradationEntity serializer to handle non-codeclimate-standard severities better to fix issues where some external linting tools (e.g. golanglint-ci) might produce a report of degradations in codeclimate format but still use severities such as warning.

There are five severities that are standardized in CodeClimate: blocker, critical, major, minor, issue, and we also add unknown to the list when sorting degradations by their severity, so when a report contains a degradation with such non-standard severity, we run into issues with displaying that information in a merge request changes tab.

See this issue and comment for more information.

Resolves #440246 (closed).

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.

How to set up and validate locally

• On your local GDK, create a new project.
• In the new project, add a .gitlab-ci.yml file with the following content:

include:
  - template: Code-Quality.gitlab-ci.yml

lint:
  stage: test
  image: golangci/golangci-lint:v1.55.2
  script:
    - golangci-lint run --issues-exit-code 0 --print-issued-lines=false --out-format code-climate:gl-code-quality-report.json,line-number

• Add some golang code with a few linting issues.
• Run a new pipeline.
• Notice how the gl-code-quality-report.json file generated by golangci-lint includes some degradations with warning severity.
• Open a merge request to update the golang code you have added previously.
• Validate the merge request changes tab located at merge_requests/:id/-/diffs load degradations successfully.