feat: bump CI version of Node to 20 (#1233)

## PR Checklist

- [x] Addresses an existing open issue: fixes #1204
- [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

Swaps the number from 18 to 20. Straightforward.

.github/actions/prepare/action.yml

@@ -8,7 +8,7 @@ runs:
     - uses: actions/setup-node@v4
       with:
         cache: pnpm
-        node-version: "18"
+        node-version: "20"
     - run: pnpm install --frozen-lockfile
       shell: bash
   using: composite

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

@@ -18,7 +18,7 @@ runs:
     - uses: actions/setup-node@v4
       with:
         cache: pnpm
-        node-version: '18'
+        node-version: '20'
     - run: pnpm install --frozen-lockfile
       shell: bash
   using: composite

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

@@ -12,7 +12,7 @@ export function createDotGitHubActions() {
 							{ uses: "pnpm/action-setup@v2" },
 							{
 								uses: "actions/setup-node@v4",
-								with: { cache: "pnpm", "node-version": "18" },
+								with: { cache: "pnpm", "node-version": "20" },
 							},
 							{
 								run: "pnpm install --frozen-lockfile",