test: skip node-fetch and @whatwg-node/fetch tests on Node 21+

[aleclarson]

The --no-experimental-fetch flag was removed in Node 21:
nodejs/node#52611

When native fetch exists, node-fetch and @whatwg-node/fetch polyfills are not used. So in Node 21+, the integration tests for those polyfills are simply testing native fetch.

Therefore, we can skip the integration tests for the node-fetch and @whatwg-node/fetch polyfills when the Node major version is 21 or higher.

[aleclarson]

CI failure appears to be unrelated to this PR

[cyco130]

I want to remove those entry points anyway. It shouldn't be Hattip's responsibility to polyfill fetch anymore (it made sense back in Node 14/16 days).

[cyco130]

Hmm, now I'm confused. My node v21.7.3. and v22.11.0 still accept --no-experimental-fetch. Maybe no need to rush it yet?

[aleclarson]

Ah, looks like they removed it in v23

[cyco130]

Well, otherwise CI would have started failing much earlier :D