chore(deps): update actions/checkout action to v6 (#47)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://github.com/actions/checkout) | action | major | `v5` -> `v6` |
| [actions/checkout](https://github.com/actions/checkout) | action | major | `v5.0.1` -> `v6.0.0` |

---

### Release Notes

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

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

[Compare Source](https://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 is behind base branch, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about these updates again.

---

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

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0Mi4xOC4wIiwidXBkYXRlZEluVmVyIjoiNDIuMTguMCIsInRhcmdldEJyYW5jaCI6Im1haW4iLCJsYWJlbHMiOlsidHlwZS9kZXBlbmRlbmNpZXMiXX0=-->

Reviewed-on: #47
Co-authored-by: renovate[bot] <renovate-bot@git.tainton.uk>
Co-committed-by: renovate[bot] <renovate-bot@git.tainton.uk>

.github/workflows/ci-python-poetry.yml

@@ -22,7 +22,7 @@ jobs:
           PYTHON_VERSION=${{ inputs.python-version }}
           echo "value=${PYTHON_VERSION:-"3.11"}" >> $GITHUB_OUTPUT
       - name: Check out repository code
-        uses: actions/checkout@v5.0.1
+        uses: actions/checkout@v6.0.0
         with:
           fetch-depth: 0
       - name: Setup Python