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 83.43%. Comparing base (964c40e) to head (fad327d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8894      +/-   ##
==========================================
+ Coverage   83.42%   83.43%   +0.01%     
==========================================
  Files         120      120              
  Lines        8097     8097              
  Branches     1944     1944              
==========================================
+ Hits         6755     6756       +1     
+ Misses       1342     1341       -1     

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