Rename @initial to @starting-style

https://github.com/w3c/csswg-drafts/issues/8174#issuecomment-1516459733

Bug: 1412851
Change-Id: Ic689460dac934b93039d1db2d6e30866470a0274
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4474048


Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147154}