Update actions/checkout action to v4 (#38)

This PR contains the following updates:

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

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### Release Notes

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

### [`v4`](https://github.com/actions/checkout/blob/HEAD/CHANGELOG.md#v410)

[Compare Source](actions/checkout@v3...v4)

-   [Add support for partial checkout filters](actions/checkout#1396)

</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 becomes conflicted, 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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Reviewed-on: https://git.apud.pl/jacek/adventofcode/pulls/38
Co-authored-by: Renovate <renovate@apud.pl>
Co-committed-by: Renovate <renovate@apud.pl>

.gitea/workflows/test.yaml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Check out repository code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Set up Python 3.11
         run: pip3 install --upgrade pip
         run: pip3 install pyenv