Update due date example for High severity issues

Why is this change being made?

Our due date example for High severity issues in our Security page is incorrect. It still says the due date as 60 days but our Vulnerability Management page has our remediation SLA set as 30 days for High severity issues. See Final version for Vulnerability SLA updates (ot... (!114057 - merged) for additional info.

/cc @gitlab-com/gl-security/appsec

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