Upgrade GitHub workflows to use latest Node.js and actions

This commit updates the Node.js versions and action versions in the GitHub workflows:

- In `npm-publish.yml`:
  - Updated Node.js to 20.x.
  - Updated `actions/checkout` to v4.
  - Updated `actions/setup-node` to v4.
- In `pull-request.yml`:
  - Updated Node.js matrix to [18.x, 20.x].
  - Updated `actions/checkout` to v4.
  - Updated `actions/setup-node` to v4.

Author: google-labs-jules[bot]

.github/workflows/npm-publish.yml

@@ -9,21 +9,21 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-node@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
         with:
-          node-version: 16
+          node-version: 20.x
       - run: npm install
       - run: npm run build
 
   publish-npm:
     needs: build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-node@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
         with:
-          node-version: 16
+          node-version: 20.x
           registry-url: https://registry.npmjs.org/
       - run: npm install
       - run: npm publish

.github/workflows/pull-request.yml

@@ -13,13 +13,13 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [12.x, 14.x, 16.x]
+        node-version: [18.x, 20.x]
         # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v2
+        uses: actions/setup-node@v4
         with:
           node-version: ${{ matrix.node-version }}
           cache: "npm"