Clarify labeling for vulnerabilities behind feature flags

Why is this change being made?

According to https://about.gitlab.com/handbook/engineering/security/#severity-and-priority-labels-on-security-issues, time based SLAs do not apply to known vulnerabilities that are feature flagged and disabled.

Unfortunately, there is no easy way to identify if a vulnerability is feature flagged and that flag is disabled.

This MR adds clarification on how to label vulnerabilities issues to identify if they are behind a feature flag and it is disabled.

Relates to https://gitlab.com/gitlab-com/ops-sub-department/ops-engineering-management/-/issues/129

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