Add auto-fix Label

What does this MR do?

This a part of a follow-up from Auto-Fix MVC it adds a special label for MRs that are created by GitLab-Security-Bot. We need it to filter such MRs and clearly indicate to users that those MRs are auto-created.

This feature is a part of a bigger feature and it's under the feature flag security_auto_fix

Screenshots (strongly suggested)

When the label is added we can filter:

And we see a note on the Vulnerability Report page

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