chore(css): remove unneeded CSS vendor prefixes (#37903)

Removes redundant/obsolete WebKit prefixes:

- `-webkit-mask-*` — duplicate the unprefixed `mask-*` siblings already
present; every supported browser handles unprefixed CSS Masking
longhands.
- `-webkit-overflow-scrolling: touch` — a no-op outside iOS Safari <13.

Browser floor (all support unprefixed `mask`): Chrome 120+, Safari
15.4+, Firefox 53+, and PaleMoon/Goanna (verified: unprefixed `mask`
longhands implemented unconditionally in UXP).

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This PR was written with the help of Claude Opus 4.8

Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>