Update upgrade paths section

What does this MR do?

The upgrade paths section has been discussed quite a few times, and some folks find it confusing.

This MR:

• Turns the upgrade paths into a list for better visibility
• Removes table of examples as it was confusing and duplicate effort

---

before	after
https://docs.gitlab.com/ee/update/#upgrade-paths	https://main-ee-109875.docs.gitlab-review.app/ee/update/index.html#upgrade-paths

Related issues

Latest discussion where the upgrade paths section confused folks #388928 (comment 1249268118).

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