Update Milestone Planning Process

Why is this change being made?

This MR updates our milestone planning process to be less of a bottleneck. The goal is for our team to be continuously shipping despite being tied to a milestone release process.

• Updates the Product Development Flow to emphasize continuous progression so we are not bottlenecked by milestone planning.
• Removes use of the workflowscheduling workflow label to avoid blocking workflow progressing by requiring a synchronization point.
• Removes use of the Stretch label to focus on Deliverables.
• Updates use of Deliverable label for all milestone commitment issues. This will allow us to track Say/Do ratio
• Adds explicit steps and dates to the Product Development Timeline
• Updates milestone planning issue sections to match recent milestone planning templates
• Updates tasks for milestone planning to match recent milestone planning templates
• Temporarily removes iteration and follow up issues section. This will be updated in a follow up issue.
• Updates heading levels to be more shallow

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
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