GitHub Actions: Transitioning from Node 16 to Node 20

[nemuvski]

Node.js 16 actions are deprecated.
Please update the following actions to use Node.js 20: actions/checkout@v3, actions/setup-node@v3, pnpm/action-setup@v2, actions/cache@v3.
For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

[nemuvski]

pnpm/action-setup@v2 に関しては corepack 利用する形にする。

- run: corepack enable
- uses: actions/setup-node@v4
  with:
    node-version: 20
    cache: "pnpm"

- name: Install dependencies
  run: pnpm install

ref: pnpm/action-setup#99 (comment)