Update actions/checkout action to v4.2.1 (#19)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://code.forgejo.org/actions/checkout) | action | minor | `v4.1.7` -> `v4.2.1` |

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### Release Notes

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

### [`v4.2.1`](https://code.forgejo.org/actions/checkout/blob/HEAD/CHANGELOG.md#v421)

[Compare Source](https://code.forgejo.org/actions/checkout/compare/v4.2.0...v4.2.1)

-   Check out other refs/\* by commit if provided, fall back to ref by [@&#8203;orhantoy](https://github.com/orhantoy) in https://github.com/actions/checkout/pull/1924

### [`v4.2.0`](https://code.forgejo.org/actions/checkout/blob/HEAD/CHANGELOG.md#v420)

[Compare Source](https://code.forgejo.org/actions/checkout/compare/v4.1.7...v4.2.0)

-   Add Ref and Commit outputs by [@&#8203;lucacome](https://github.com/lucacome) in https://github.com/actions/checkout/pull/1180
-   Dependency updates by [@&#8203;dependabot-](https://github.com/dependabot-) https://github.com/actions/checkout/pull/1777, https://github.com/actions/checkout/pull/1872

</details>

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### Configuration

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

🚦 **Automerge**: Enabled.

♻ **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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This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNy4zNDAuMyIsInVwZGF0ZWRJblZlciI6IjM3LjM0MC4zIiwidGFyZ2V0QnJhbmNoIjoibWFpbiIsImxhYmVscyI6W119-->

Reviewed-on: #19
Co-authored-by: Johnny5 <bot@snaile.de>
Co-committed-by: Johnny5 <bot@snaile.de>

.forgejo/workflows/renovate.yml

@@ -17,7 +17,7 @@ jobs:
       RENOVATE_GIT_AUTHOR: "Johnny5 <bot@snaile.de>"
 
     steps:
-      - uses: https://code.forgejo.org/actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+      - uses: https://code.forgejo.org/actions/checkout@eef61447b9ff4aafe5dcd4e0bbf5d482be7e7871 # v4.2.1
 
       - name: Renovate
         run: renovate ${{ github.repository }}