chore: Update sass and change colour syntax

[mister-ben]

Description

Projects using newer versions of sass which have deprecated lighten() will get deprecation warnings.

This updates sass, and switches to color.adjust(). This function is available since dart sass 1.23.0.

Note this would increase the output CSS size, as it will output different colour syntax, .e.g.

<   background-image: linear-gradient(-180deg, #fff 88%, rgb(114.9141509434, 132.7028301887, 159.3858490566) 100%);
---
>   background-image: linear-gradient(-180deg, #fff 88%, #73859f 100%);

Closes #8876

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.25%. Comparing base (077077b) to head (26918f8).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8894      +/-   ##
==========================================
+ Coverage   83.21%   83.25%   +0.03%     
==========================================
  Files         120      120              
  Lines        8068     8097      +29     
  Branches     1938     1944       +6     
==========================================
+ Hits         6714     6741      +27     
- Misses       1354     1356       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.