silence sass `legacy-js-api` warning (#72632)
### Why?
This PR silences `legacy-js-api` warning enabled from PR
https://github.com/vercel/next.js/pull/70067 by updating the
`sass-loader` to v15. Will follow up with adding an option to enable the
sass-loader v16 by PR https://github.com/vercel/next.js/pull/72423.
x-ref: https://github.com/vercel/next.js/issues/71638
