* Add case insensitive match for severity

```text
NoMethodError: undefined method `each' for nil:NilClass
  from gitlab/ci/parsers/security/common.rb:14:in `parse!'
  from ee/ci/build.rb:118:in `parse_security_artifact_blob'
  from ee/ci/build.rb:57:in `block (2 levels) in collect_security_report
  from ee/ci/build.rb:54:in `tap'
  from ee/ci/build.rb:54:in `block in collect_security_reports!'
  from ci/build.rb:828:in `block (2 levels) in each_report'
  from gitlab/ci/build/artifacts/adapters/raw_stream.rb:22:in `each_blob
  from ci/job_artifact.rb:183:in `block in each_blob'
  from gitlab_uploader.rb:96:in `open'
```
https://sentry.gitlab.net/gitlab/gitlabcom/issues/991284/events/20532078

```text
NoMethodError: undefined method `[]' for nil:NilClass
  from gitlab/ci/parsers/security/dependency_scanning.rb:16:in `create_
  from gitlab/ci/parsers/security/common.rb:53:in `create_vulnerability
  from gitlab/ci/parsers/security/common.rb:15:in `block in parse!'
  from gitlab/ci/parsers/security/common.rb:14:in `each'
  from gitlab/ci/parsers/security/common.rb:14:in `parse!'
  from ee/ci/build.rb:116:in `parse_security_artifact_blob'
  from ee/ci/build.rb:55:in `block (2 levels) in collect_security_report
  from ee/ci/build.rb:52:in `tap'
  from ee/ci/build.rb:52:in `block in collect_security_reports!'
  from ci/build.rb:828:in `block (2 levels) in each_report'
```
https://sentry.gitlab.net/gitlab/gitlabcom/issues/972642/events/20413167

```text
TypeError: no implicit conversion of nil into String
  from gitlab/ci/reports/security/occurrence.rb:78:in `digest'
  from gitlab/ci/reports/security/occurrence.rb:78:in `hexdigest'
  from gitlab/ci/reports/security/occurrence.rb:78:in `generate_project_
  from gitlab/ci/reports/security/occurrence.rb:37:in `initialize'
  from gitlab/ci/parsers/security/common.rb:48:in `new'
  from gitlab/ci/parsers/security/common.rb:48:in `create_vulnerability'
  from gitlab/ci/parsers/security/common.rb:15:in `block in parse!'
  from gitlab/ci/parsers/security/common.rb:14:in `each'
  from gitlab/ci/parsers/security/common.rb:14:in `parse!'
  from ee/ci/build.rb:113:in `parse_security_artifact_blob'
  from ee/ci/build.rb:64:in `block (2 levels) in collect_security_report
  from ee/ci/build.rb:61:in `tap'
  from ee/ci/build.rb:61:in `block in collect_security_reports!'
  from ci/build.rb:821:in `block (2 levels) in each_report'
```

https://sentry.gitlab.net/gitlab/gitlabcom/issues/903549/events/18937780