Explain why Support doesn't take action on customer needs in Slack; remove Support Slack channels list

Why is this change being made?

Per the discussion in gitlab-com/support/support-team-meta#4768 (closed), this MR adds a list of reasons why Support does not act on customer-specific requests in Slack. It also removes the list of Support Slack channels at the bottom of the page, since there is no clear reason why teams outside of Support would need a general list of those channels.

Adding a list of explanations for the policy may help reduce the frequency of team members outside of Support making customer-specific requests in Slack.

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.
• 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
• 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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