ci: drop Node.js 18, add 20/22, update pnpm to v10

- Node.js 18 reached EOL and vitest 4 requires Node.js 20+
- Add Node.js 20 (maintenance LTS) and 22 (active LTS)
- Update pnpm from v8 to v10 to match local environment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: yamatatsu

.github/workflows/ci.yml

@@ -13,7 +13,8 @@ jobs:
       fail-fast: false
       matrix:
         node-version:
-          - 18
+          - 20
+          - 22
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -25,7 +26,7 @@ jobs:
         name: Install pnpm
         id: pnpm-install
         with:
-          version: 8
+          version: 10
           run_install: false
       - run: pnpm install
       - run: pnpm test run