Update estimation and milestone process product intelligence

Why is this change being made?

Product Intelligence agreed within their Retro issue that the estimation and milestone planning process need some changes. The discussed changes are applied to the Product Intelligence Group page in this MR:

• 2 instead of 3 estimations necessary for the agreement
• Estimations being done in bulk during the second to last week before a new milestone starts to provide feedback to PM/EM early enough.
• Milestone process streamlined to only 2 phases with estimation happening before next milestone starts

P.S.: The most important thing I discovered during this MR -> there is a difference between weighting and weighing and only the second word is appropriate in this context

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
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• 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
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