Add link to metrics information of Gemnasium DB update frequency

Problem to solve

We currently document our update rate in https://docs.gitlab.com/ee/user/application_security/#maintenance-and-update-of-the-vulnerabilities-database. The comment "The Gemnasium DB is updated at least once a week." states the update rate. That being said, we want to point back to the metrics at https://about.gitlab.com/handbook/engineering/development/performance-indicators/#cve-issue-to-update to reflect what our current mean and median are.

Further details

Secure products are affected.

Affected section: https://docs.gitlab.com/ee/user/application_security/#maintenance-and-update-of-the-vulnerabilities-database

Proposal

Add a line with something to the effect "Our current measurement of time from CVE being issued to our product being updated is recorded at https://about.gitlab.com/handbook/engineering/development/performance-indicators/#cve-issue-to-update" - with appropriate hyperlinking.

Who can address the issue

Anyone in the docs or development teams.

Other links/references

None