[scroll-animations] Modernize at-scroll-timeline-...-endpoint.html

Bug: 1317765
Change-Id: Ia676738daad3d269fcd37244ac87aa52d47be69f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3856584


Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#10406026}