Pass same mangling option as terser to SWC minifier (#56281)
### What?
Pass `keep_classnames: true` to the SWC minifier, as we are passing it to `terser`.
### Why?
https://github.com/vercel/next.js/blob/5f7faac1e676be0819dd5d828bffed8a7105a78e/packages/next/src/build/webpack-config.ts#L953-L958
### How?
Closes WEB-1676
Fixes #55682
