[ci] bump CI Node.js version to 24.21.0 #37818

Node.js 20 is EOL.

Also fixes two tests affected by Node 24:
- Check pooled buffers against Buffer.poolSize instead of a hard-coded 8 KiB, while asserting the backing buffer is larger than the input chunk.
- Remove AsyncLocalStorage's defaultValue: null from the server task test so an absent task remains undefined; Node 24 now honors that option.

Test Plan:
CI