[scroll-animations] Broader scope of scroll timelines

[mozilla-apprentice]

A resolution was made for csswg-drafts/#7759.

**[scroll-animations] Broader scope of scroll timelines **

• RESOLVED: Reduce default scoping to ancestors only, add scroll-timeline-attachment as proposed in the issue

Discussion.

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If no bug is needed, the issue can be closed.

[mozilla-apprentice]

https://bugzilla.mozilla.org/show_bug.cgi?id=1823500