fix(ci): pin lockfile to next 16.2.3 to fix MDX prerender crash

Regenerating the lockfile from scratch bumped next to 16.2.10, which crashes the
Server Components prerender of MDX pages (/deploy/how-it-works,
/deploy/build-requirements). Restore the known-good next 16.2.3 tree and layer
the new devDeps on top, keeping a complete (npm ci-consistent) lockfile.

Author: boyander