update PI metrics

Why is this change being made?

Changes post-review gitlab-org/quality/engineering-analytics/team-tasks#134

• Past due security issues
  • Update health reason: "* Security issues continue to be an area of focus as requirements have become more stringent. * Large decreases due to security releases, large increases due to new scanners."
• Open MR Review Time (OMRT)
  • Change to "* Normal increase and recovery for holidays taken effect. * Plan to change to all MRs (as opposed to MRs authored by Development"
• Development Average Age of Open Positions
  • Change to "* We do not anticipate improvement until existing positions are closed."
• Development Department Discretionary Bonus Rate
  • Change first line to "* We have been consistently below 10% goal for the last 6 months."
• Remove MR Rate Percentiles
• Remove Open to Merge
• Overall MRs by Type
  • Change first line to "* Have worked to remove undefined MRs."
• Remove GitLab.com Availability Apdex SLO Trends
• Remove CI Runners Apdex SLO Trends

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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