chore(deps): update https://code.forgejo.org/actions/checkout action to v6 (#272)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [https://code.forgejo.org/actions/checkout](https://code.forgejo.org/actions/checkout) | action | major | `v4` -> `v6` |

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### Release Notes

<details>
<summary>actions/checkout (https://code.forgejo.org/actions/checkout)</summary>

### [`v6`](https://code.forgejo.org/actions/checkout/blob/HEAD/CHANGELOG.md#v600)

[Compare Source](https://code.forgejo.org/actions/checkout/compare/v5...v6)

- Persist creds to a separate file by [@&#8203;ericsciple](https://github.com/ericsciple) in [#&#8203;2286](actions/checkout#2286)
- Update README to include Node.js 24 support details and requirements by [@&#8203;salmanmkc](https://github.com/salmanmkc) in [#&#8203;2248](actions/checkout#2248)

### [`v5`](https://code.forgejo.org/actions/checkout/blob/HEAD/CHANGELOG.md#v501)

[Compare Source](https://code.forgejo.org/actions/checkout/compare/v4...v5)

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

</details>

---

### Configuration

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This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: https://git.nadeko.net/Fijxu/patchy/pulls/272
Co-authored-by: Renovate Bot <git@nadeko.net>
Co-committed-by: Renovate Bot <git@nadeko.net>

Author: Renovate Bot

.forgejo/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: runner
 
     steps:
-    - uses: https://code.forgejo.org/actions/checkout@v4
+    - uses: https://code.forgejo.org/actions/checkout@v6
 
     - uses: https://code.forgejo.org/docker/setup-buildx-action@v3
       name: Setup Docker BuildX system