Handle mixed-decls deprecation being silenced

[jacobo-dominguez-wgu]

This warning is showing on profile mfe during conversion to frontend base:
Module Warning (from ./node_modules/sass-loader/dist/cjs.js): mixed-decls deprecation is obsolete. If you were previously silencing it, your code may now behave in unexpected ways.

This mixed-decls is being also silenced on severals projects in the org https://github.com/search?q=org%3Aopenedx%20mixed-decls&type=code.

To deal with this warning it is recommended to:

1. Update your Sass code: Review your .scss or .sass files for places where you mix variable, function, or mixin declarations in the same scope. Separate them as recommended by Sass best practices.
2. Update dependencies: Make sure you are using the latest versions of sass, sass-loader, and related packages.
3. Remove silencing config: If you previously silenced this warning in your config, remove that configuration.

Mixed-decls breaking change docs: https://sass-lang.com/documentation/breaking-changes/mixed-decls/

[jacobo-dominguez-wgu]

FYI @arbrandes, @brian-smith-tcril.