upgrade: update pnpm to v8.15.8 (#511)

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

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [pnpm](https://pnpm.io) ([source](https://togithub.com/pnpm/pnpm)) |
[`8.15.7` ->
`8.15.8`](https://renovatebot.com/diffs/npm/pnpm/8.15.7/8.15.8) |
[![age](https://developer.mend.io/api/mc/badges/age/npm/pnpm/8.15.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/npm/pnpm/8.15.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/npm/pnpm/8.15.7/8.15.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/npm/pnpm/8.15.7/8.15.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

---

### Release Notes

<details>
<summary>pnpm/pnpm (pnpm)</summary>

###
[`v8.15.8`](https://togithub.com/pnpm/pnpm/compare/v8.15.7...v8.15.8)

[Compare
Source](https://togithub.com/pnpm/pnpm/compare/v8.15.7...v8.15.8)

</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:eyJjcmVhdGVkSW5WZXIiOiIzNy4zMzEuMCIsInVwZGF0ZWRJblZlciI6IjM3LjMzMS4wIiwidGFyZ2V0QnJhbmNoIjoibWFpbiIsImxhYmVscyI6W119-->

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

Author: renovate[bot]

package.json

@@ -17,5 +17,5 @@
     "prettier": "^3.0.3",
     "prettier-plugin-packagejson": "^2.4.5"
   },
-  "packageManager": "[email protected].7"
+  "packageManager": "[email protected].8"
 }