This PR removes the modern mode experiment because:

- It does not yield meaningful bundle size wins when compared to other initiatives we've taken
- It's not compatible with webpack 5 (which we're upgrading to)
- It's currently broken and causes most apps to malfunction
- There's no champion currently owning the experiment

We can re-introduce this in the future when we'd like to make it a default for all Next.js apps.

Note: **Next.js still supports Differential Loading (`nomodule`) and does it by default.** This PR strictly removes the experimental modern _syntax_, and does not disable our existing modern/legacy polyfilling.

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Fixes #19200
Fixes #18960
Fixes #14707
Fixes #14465