Update vulnerability triage process to include unused dependency removal

Why is this change being made?

This change is being made to document the information shared in https://gitlab.com/gitlab-org/gitlab/-/issues/366560#note_1047299052. Often times, it's not possible to upgrade a dependency and as an alternative it may be possible to remove it if it's unused. This MR documents the OS specific guidance to aid in determining if a dependency can be safely removed. Removing unused dependencies will help reduce the amount of vulnerabilities and decrease the attack surface of the products.

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