Add detected_at field to ES vulnerability index

What does this MR do and why?

References

• Related issue: #578070
• Main parent issue: #577550

I have used Resolve "Add risk_score field to vulnerability ... (!206657 - merged) • Schmil Monderer • 18.5 as an example MR

Documentation

• https://docs.gitlab.com/development/testing_guide/best_practices/#elasticsearch-specs

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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Related to #578070