Follow-up from "Introduce vulnerabilities trends chart"

The following discussion from !46591 (merged) should be addressed:

• @peterhegman started a discussion: (+2 comments)

  suggestion: I think we may be trying to move away from createFlash. Do you know how hard it would be to use GlAlert instead here?

Implementation plan

• frontend Replace createFlash instances in ee/app/assets/javascripts/security_dashboard/components/project_security_charts.vue with GlAlert.
• frontend Update tests