Update actions/checkout action to v6 (#965)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://redirect.github.com/actions/checkout) |
action | major | `v5` → `v6` |

---

### Release Notes

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

### [`v6`](https://redirect.github.com/actions/checkout/compare/v5...v6)

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

</details>

---

### 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.

---

- [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check
this box

---

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View the [repository job
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Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/pr.yml

@@ -52,7 +52,7 @@ jobs:
     needs: pre-commit
     steps:
       - name: Checkout the code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 1
       - name: Test applying patch