fix(apiSmokeTest): downgrade undici resolution to 6.21.1 for Node 20 compat

The previous resolution pinned undici to 8.2.0. undici 8.x requires
Node 22+ (uses webidl.util.markAsUncloneable, only available on the
runner's bundled Node 22+). The apiSmokeTest action declares
`using: node20`, so on the runner the dist loads on Node 20 and fails
at module-evaluation time with:

  TypeError: webidl.util.markAsUncloneable is not a function
    at new CacheStorage (.../dist/index.js:...)

This was masked in the self-tests of #55 (they passed) but surfaced
when nshm-toshi-api's deploy-test ran against the merged main.

Pin to undici 6.21.1 — actively maintained, Node 18+ compatible,
matches what the older pre-#55 dist effectively bundled. Verified
locally that the rebuilt dist loads cleanly on Node 20.19.2.

dist/index.js + sourcemap rebuilt via `yarn build`.

Author: chrisbc