Use "sass:list" for append function

[p8]

Description

Calling global append is deprecated. This uses "sass:list" instead.

Types of changes

• [ ] Documentation
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (anything that would change an existing functionality)
• [x] Maintenance (refactor, code cleaning, development tools...)

Checklist

• [x] I have read and follow the CONTRIBUTING.md document.
• [x] The pull request title and template are correctly filled.
• [x] The pull request targets the right branch (develop or develop-v...).
• [x] My commits are correctly titled and contain all relevant information.
• [x] I have updated the documentation accordingly to my changes (if relevant).
• [x] I have added tests to cover my changes (if relevant).

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[javierjulio]

@p8 thank you so much for doing this and the other PR's! They all look great.

@joeworkman can you please review this PR and the others in: #15520, #15521, #15522, #15523, #15524 which all resolve the reported SASS deprecations (excluding the @import case)?