Update actions/setup-node action to v6 (#56)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/setup-node](https://github.com/actions/setup-node) | action | major | `v4` -> `v6` |

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### Release Notes

<details>
<summary>actions/setup-node (actions/setup-node)</summary>

### [`v6`](https://github.com/actions/setup-node/compare/v5...v6)

[Compare Source](https://github.com/actions/setup-node/compare/v5...v6)

### [`v5`](https://github.com/actions/setup-node/compare/v4...v5)

[Compare Source](https://github.com/actions/setup-node/compare/v4...v5)

</details>

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### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

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 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

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This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: #56
Co-authored-by: Renovate Bot <renovate-bot@alexlebens.net>
Co-committed-by: Renovate Bot <renovate-bot@alexlebens.net>

.gitea/workflows/test-build.yaml

@@ -22,7 +22,7 @@ jobs:
           version: 10.x
 
       - name: Set up Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: 22.21.1
           cache: pnpm