Put a link to find the upcoming security release issue easily

What does this MR do and why?

It updates the Security Developer Workflow template to add a link to find the upcoming security release more easily. Going to https://gitlab.com/gitlab-org/gitlab/-/issues?sort=created_date&state=opened&label_name[]=upcoming+security+release is quicker than checking the #release channel in Slack when you're opening the issue.

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MR acceptance checklist

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