fix: upgrade pnpm/action-setup to v4 (#1620)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #1619
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

<!-- Description of what is changed and how the code change does that.
-->

This PR upgrades the `pnpm/action-setup@v2` to `pnpm/action-setup@v4`
which should resolve the warning.

From the looks of the releases page, 3 would work as well:
https://github.com/pnpm/action-setup/releases

Author: johnnyreilly

.github/actions/prepare/action.yml

@@ -4,7 +4,7 @@ name: Prepare
 
 runs:
   steps:
-    - uses: pnpm/action-setup@v2
+    - uses: pnpm/action-setup@v4
       with:
         version: 9
     - uses: actions/setup-node@v4

src/steps/writing/creation/dotGitHub/actions.test.ts

@@ -14,7 +14,7 @@ name: Prepare
 
 runs:
   steps:
-    - uses: pnpm/action-setup@v2
+    - uses: pnpm/action-setup@v4
       with:
         version: 9
     - uses: actions/setup-node@v4

src/steps/writing/creation/dotGitHub/actions.ts

@@ -10,7 +10,7 @@ export function createDotGitHubActions() {
 					runs: {
 						steps: [
 							{
-								uses: "pnpm/action-setup@v2",
+								uses: "pnpm/action-setup@v4",
 								with: { version: 9 },
 							},
 							{