ci: test on node 20.5 rather than 20.6

[danielroe]

🔗 Linked issue

#23020
nodejs/node#49497

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This runs our CI on node 20.5 to avoid breakage in node 20.6 (example failed run: https://github.com/nuxt/nuxt/actions/runs/6096011429/job/16540685059?pr=23040).

Upstream solution has already been merged but I imagine it will likely take a while to filter back down to GHA runners, even after a hotfix release.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[stafyniaksacha]

@danielroe Node 20.6.1 seem to have fixed it: https://github.com/nodejs/node/releases/tag/v20.6.1
Maybe worth to use it?

[danielroe]

it might take a moment before it's rolled out to github actions with node-version: 20, but I'll open a PR to check.