Adjust Color of Low Severity Symbol

What does this MR do?

This MR changes the color of the vulnerability low severity symbol from 300 to 200 (text-warning).

Relates to #247538 (closed).

Screenshots

Component	Before	After
"Vulnerabilities over time" on instance- and group-level security dashboards
An entry in the vulnerability list
Vulnerability stats on project-level security dashboard

I might have missed places where the icon is used (I don't think so though) but there is no other occurrence of the text-warning-300 class in the code base.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team