chore(deps): update actions/checkout action to v4 (#57)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://togithub.com/actions/checkout) | action |
major | `v3` -> `v4` |

---

### Release Notes

<details>
<summary>actions/checkout (actions/checkout)</summary>

###
[`v4`](https://togithub.com/actions/checkout/blob/HEAD/CHANGELOG.md#v400)

[Compare Source](https://togithub.com/actions/checkout/compare/v3...v4)

- [Support fetching without the --progress
option](https://togithub.com/actions/checkout/pull/1067)
-   [Update to node20](https://togithub.com/actions/checkout/pull/1436)

</details>

---

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are satisfied.

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rebase/retry checkbox.

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---

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.github/workflows/sync.yml

@@ -21,7 +21,7 @@ jobs:
       id: date
       run: echo "date=$(date +'%Y-%m-%d')" >> "$GITHUB_OUTPUT"
     - name: Checkout code
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
     - name: GitHub user
       run: |
         # https://api.github.com/users/github-actions[bot]