chore(deps): update actions/cache action to v5 (#403)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/cache](https://github.com/actions/cache) | action | major | `v4` -> `v5` |

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### Release Notes

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

### [`v5`](https://github.com/actions/cache/compare/v4...v5)

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

</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 this update 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).
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Reviewed-on: #403
Co-authored-by: renovate[bot] <renovate-bot@git.tainton.uk>
Co-committed-by: renovate[bot] <renovate-bot@git.tainton.uk>

.gitea/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
           python-version: 3.14
 
       - name: uv cache
-        uses: actions/cache@v4
+        uses: actions/cache@v5
         with:
           path: /tmp/.uv-cache
           key: uv-${{ runner.os }}-${{ hashFiles('uv.lock') }}