Consider replacing deprecated `clip`

Description

The visually-hidden helper includes an outdated syntax of clip: rect() that may need replacing with some other clipping properties or maybe updating the helper to something more current overall.

Steps to reproduce

The next stylelint-config-standard-scss major version won't like it.

😭 /actions/runs/17982808350/job/51153182926 ✂️

Expected result

https://developer.mozilla.org/en-US/docs/Web/CSS/clip

Actual result

https://github.com/mozilla/protocol/blob/81e6c70430416c3cde7c088f74c0e9fb9e1500a4/assets/sass/protocol/includes/mixins/_utils.scss#L128-L133

Environment

This might be breaking of sorts, in case a consumer applies the mixin, but then e.g. redefines or undoes the visual hiding in some cases, changing the clip value there — so there might be a migration to the newer mixin content version necessary for them.

• https://github.com/mozilla/bedrock/blob/2025-09-24/media/css/m24/components/footer-refresh.scss#L412
• https://github.com/mozilla/bedrock/blob/2025-09-24/media/css/firefox/whatsnew/whatsnew-142.scss#L1564

Start with the visually-hidden helper in assets/sass/protocol/includes/mixins/_utils.scss at the referenced lines, then review the stylelint-config-standard-scss failure from the linked workflow. Check the linked Mozilla bedrock usages for assumptions about the current clip value. Done means the deprecated syntax is addressed, lint passes, and any consumer migration impact is documented.