Resolve "Refresh health status for Development KPIs"

Why is this change being made?

Refresh health status for Development KPIs beginning of January 2021

• Development Handbook Update Frequency Rate
  • The target was updated to 0.5 in this MR.
  • Increase in December
  • Changed metric name to Development Overall Handbook Update Frequency Rate
  • Add clarification on calculation of rate
• Review To Merge Time (RTMT)
  • Increase in December, likely due to holidays and F&F day
  • Decrease RTMT target by 0.5 days
• Development Department Narrow MR Rate
  • Decrease in December, likely due to holidays and F&F day

Review link: https://10275-refresh-health-status-for-development-kpis.about.gitlab-review.app/handbook/engineering/development/performance-indicators/

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
• Assign this change to the correct 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 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 relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)

Closes #10275 (closed)