[Observable change] Simplify unmount process

[nolanlawson]

To simplify the component unmounting process, #2709 also tried to make an observable change to the order of disconnectedCallback events. This applies even in native shadow DOM and with native custom element lifecycle callbacks, because it's about how we remove elements from the DOM during unmounting.

Since it's an observable change, we may have to ship it with API versioning.

[nolanlawson]

Duplicate of #2713