upgrade: update actions/download-artifact action to v4 (#461)

[![Mend
Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
|
[actions/download-artifact](https://togithub.com/actions/download-artifact)
| action | major | `v3` -> `v4` |

---

### Release Notes

<details>
<summary>actions/download-artifact (actions/download-artifact)</summary>

###
[`v4`](https://togithub.com/actions/download-artifact/compare/v3...v4)

[Compare
Source](https://togithub.com/actions/download-artifact/compare/v3...v4)

</details>

---

### Configuration

📅 **Schedule**: Branch creation - "every weekend" in timezone
Asia/Tokyo, 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

---

This PR has been generated by [Mend
Renovate](https://www.mend.io/free-developer-tools/renovate/). View
repository job log
[here](https://developer.mend.io/github/FlutterGen/flutter_gen).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNy4xMDMuMSIsInVwZGF0ZWRJblZlciI6IjM3LjEwMy4xIiwidGFyZ2V0QnJhbmNoIjoibWFpbiJ9-->

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

Author: renovate[bot]

.github/workflows/release.yml

@@ -94,7 +94,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: build
     steps:
-      - uses: actions/download-artifact@v3
+      - uses: actions/download-artifact@v4
         with:
           name: sha256