Update global prioritization

Why is this change being made?

Currently we have too many schemes to have global prioritization. We need to unify these efforts. This MR takes that alignment.

This MR:

• Gets rid of Multi-group swarming
• Make's realignments point to the Peopleops page (which are the owners)
• Replaces headcount resets with Borrow
• Removes the plan listed in headcount resets (as it's for transitioning to a new-hire specifically)
• Explains when to use rapid action or Reprioritization or Realignment

Author Checklist

• Provided a concise title for the 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 change to the correct DRI(s)
  • 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 in 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.

Needed approvals

• @fseifoddini
• @adawar

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