Update DRI in new prioritization framework

Why is this change being made?

Based on feedback, this proposal sets the PM as the DRI for the maintenance and bug percentages of the milestone. Note, that this will be in partnership with their EM and Quality counterparts. Feedback thus far includes:

• PMs not understanding the why on the items being prioritized for bugs and maintenance
• EMs do not regularly interact with customers and internal stakeholders and could be missing critical details
• PM job families outline that the PM is the DRI for prioritization
• EMs have identified PM prioritization works for these areas today and this new prioritization should focus on fostering even better collaboration

These changes remove any artificial silos from being formed in our new prioritization proposal and reinforce our Collaboration value, especially when working with stable counterparts.

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.

---