Update team member workflow section

Why is this change being made?

Based on the async discussion we had on how to better plan our milestones, this handbook change updates how team members should interpret which issues to prioritize.

In summary, we will be switching to this issue board which does not have a milestone filter. Instead, issues belonging to the milestone will have a Deliverable label and will be stack-ranked at the top of the workflowready for development column. Team members looking for work can simply take the next issue off of the top of that column, and Deliverable issues will be prioritized first based on this method.

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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