Remove section about an option to report phishing manually

Why is this change being made?

The handbook leads one to believe that if a team member does not use the gmail client, that there is an option to manually report phishing attempts by forwarding the email to phishing@gitlab.com.

I don't use the gmail client and used the manual option following the handbook guidance, only to receive an automated response telling me to use PhishAlarm. This MR updates the handbook so it is clear that reporting via PhishAlarm is the only option. This is more efficient and will save team members time and steps when reporting a suspected phishing attempt.

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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