Fix SCSS deprecations

What does this MR do and why?

Follow-up from !4895 (comment 1999084712)

Clears up some deprecation warnings introduced in the last round of dependency updates (!4895 (merged)).

These were not causing actual issues but do add noise to the logs.

Screenshots, screen recordings, or links to review app

No visual changes expected.

• Before: Deprecation warnings: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/7346510976/viewer#L3587
• After: Pipeline with the deprecations fixed: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/7345472437/viewer#L3585

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Run yarn compile:css and verify there are not deprecation warnings from Sass
3. Browse the site and check for visual regressions

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.