ci(ci): add node 26 to test matrix

[Fdawgs]

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[gurgunday]

lgtm

[metcoder95]

This is due nodejs/node#62083, and yargs already landed a fix in v18 but c8 still uses v17.

We can chose to wait or just apply an override for c8:

  "overrides": {
    "c8": {
      "yargs": "^18.0.0"
    }
  },

I lean on the former for it.

[Fdawgs]

We can chose to wait or just apply an override for c8:

Happy to wait tbh rather than having to apply this override to every fastify repo and then removing it once c8 is updated.