Align the development, kick off and release processes

Why this change is needed

Milestones begin on the 8th, but with removal of the hard freeze on the 7th, there is now nearly a two week overlap between when the milestone ends from a planning perspective and when it ends from a release perspective (after this date, it won't be in the corresponding GitLab release). This makes it hard to reason about capacity and communicate the anticipated release for an issue.

What changed?

This commit re-aligns the two, by starting with the assumption that the kick off and release post are the start and finish of the process.

Key changes

• kick off date is now on the 18th
• development starts on the 18th and ends on the 17th which is approximately when the last version is deployed to GitLab.com - but there is no hard defined freeze because of auto deploy
• add plain English relative date descriptions (e.g. at least 5 days before ...) for easier comprehension
• explicitly mention and link to auto deploy process and feature flags
• remove duplication of this list by linking to it from the Product handbook page
• milestone closed date is now end of day on the 17th

Not changed

• release date

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Diagram