Update file hackerone-process.html.md to specify what to do regarding vulnerabilities on deprecated features.

Why is this change being made?

I'm not sure if you are going to view this as beneficial or if it just clutters the Handbook, feel free to close without merging if you think it adds unnecessary text. I personally tried to CTRL+F for deprecated feature as it came up in the handbook and could not find an answer. I thought it could be good to add that we triage them normally as per this discussion: https://gitlab.com/gitlab-com/gl-security/appsec/appsec-team/-/issues/336

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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