Update actions/checkout action to v6 (#118)

This PR contains the following updates:

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

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### Release Notes

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

### [`v6`](https://github.com/actions/checkout/blob/HEAD/CHANGELOG.md#v602)

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

- Fix tag handling: preserve annotations and explicit fetch-tags by [@&#8203;ericsciple](https://github.com/ericsciple) in [#&#8203;2356](https://github.com/actions/checkout/pull/2356)

### [`v5`](https://github.com/actions/checkout/blob/HEAD/CHANGELOG.md#v501)

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

- Port v6 cleanup to v5 by [@&#8203;ericsciple](https://github.com/ericsciple) in [#&#8203;2301](https://github.com/actions/checkout/pull/2301)

</details>

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Reviewed-on: #118
Co-authored-by: Renovate <no-reply@git.hamburg.ccc.de>
Co-committed-by: Renovate <no-reply@git.hamburg.ccc.de>

.forgejo/workflows/deploy.yaml

@@ -29,7 +29,7 @@ jobs:
           # For uploading.
           apk add rsync openssh
 
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0 # pull full history for page lastmod by git commit date