Update PIs so that MR rate includes all authors

Why is this change being made?

Background discussion in gitlab-org/quality/engineering-analytics/team-tasks#51. From the original issue:

We want to move back to calculating MR Rate as a whole rather than based on authorship. The current thinking is to move to (All MRs) / (# of members in the Development Team).

Two main Performance indicators have been modified in the handbook page for Development:

• Development Department MR rate
• Open MR Review Time ("OMRT")

What is not included in this change?

Changes to the Sisense graphs in the Handbook have not been made yet. The new version of the affected data visualizations can be found in new Sisense dashboards:

• https://app.periscopedata.com/app/gitlab/1095618/DRAFT:-Eng-Analytics-51---Development-Department-MR-Rate
• https://app.periscopedata.com/app/gitlab/1095672/DRAFT:-Eng-Analytics-51---New-Open-MR-Review-Time-(OMRT)

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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