Update actions/checkout action to v6 (#117)

This PR contains the following updates:

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

---

### Release Notes

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

### [`v6`](https://github.com/actions/checkout/blob/HEAD/CHANGELOG.md#v600)

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

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

### [`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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---

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---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0Mi43MS4wIiwidXBkYXRlZEluVmVyIjoiNDIuNzEuMCIsInRhcmdldEJyYW5jaCI6Im1haW4iLCJsYWJlbHMiOltdfQ==-->

Reviewed-on: https://code.forgejo.org/actions/forgejo-release/pulls/117
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.code.forgejo.org>
Co-authored-by: Renovate Bot <bot@kriese.eu>
Co-committed-by: Renovate Bot <bot@kriese.eu>

.forgejo/workflows/integration.yml

@@ -9,7 +9,7 @@ jobs:
   integration:
     runs-on: lxc-trixie
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - id: forgejo
         uses: https://code.forgejo.org/actions/setup-forgejo@v3.1.0

.forgejo/workflows/tests.yml

@@ -3,7 +3,7 @@ jobs:
   tests:
     runs-on: docker
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Unit tests of the supporting script forgejo-release.sh
         run: |

testdata/nested-upload-download/.forgejo/workflows/test.yml

@@ -14,7 +14,7 @@ jobs:
   setup-forgejo:
     runs-on: self-hosted
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - id: forgejo
         uses: https://code.forgejo.org/actions/setup-forgejo@v3.1.0
         with:

testdata/upload-download/.forgejo/workflows/gpg-no-passphrase.yml

@@ -92,7 +92,7 @@ jobs:
     container:
       image: 'data.forgejo.org/oci/node:24-bookworm'
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - if: ${{ !startsWith(github.ref, 'refs/tags/v') }}
         id: release-upload
         uses: SELF@vTest

testdata/upload-download/.forgejo/workflows/gpg.yml

@@ -27,7 +27,7 @@ jobs:
   upload-gpg:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - if: ${{ !startsWith(github.ref, 'refs/tags/v') }}
         id: release-upload
         uses: SELF@vTest

testdata/upload-download/.forgejo/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
   upload-download:
     runs-on: lxc-bookworm
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - if: ${{ !startsWith(github.ref, 'refs/tags/v') }}
         id: release-upload
         uses: SELF@vTest