[playground] Downgrade Next.js to a secure version (#35317)

Was bumped to a canary in https://github.com/facebook/react/pull/34499/
which got never released as stable.

Presumeably to use `Activity` which only made it into Activity in later
Next.js releases. However, `Activity` never ended up being used due to
incompatibilities with Monaco Editor. Downgrading should be safe.

Downgrading to fix
https://github.com/vercel/next.js/security/advisories/GHSA-9qr9-h5gf-34mp.
This will allow new deploys since Vercel is currently blocking new
deploys of unsafe version

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Co-authored-by: Eugene Choi <[email protected]>