Build: Improve security of the GitHub Actions setup

Changes:
1. Reference concrete hashes of actions with versions in comments
2. Update actions to recent versions
3. Set up automatic dependabot montly PRs updating actions

Fortunately, hashes with versions in comments is syntax recognized by dependabot
automatic updates; we're using it with success in jQuery Core.

Closes gh-497

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: monthly

.github/workflows/node.js.yml

@@ -12,18 +12,18 @@ jobs:
         NODE_VERSION: [10.x, 18.x]
     steps:
     - name: Checkout
-      uses: actions/checkout@v2
+      uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
 
     - name: Cache
-      uses: actions/cache@v2
+      uses: actions/cache@704facf57e6136b1bc63b828d79edcd491f0ee84 # v3.3.2
       with:
         path: ~/.npm
         key: ${{ runner.os }}-node-${{ matrix.NODE_VERSION }}-npm-lock-${{ hashFiles('**/package-lock.json') }}
         restore-keys: |
           ${{ runner.os }}-node-${{ matrix.NODE_VERSION }}-npm-lock-
 
     - name: Use Node.js ${{ matrix.NODE_VERSION }}
-      uses: actions/setup-node@v2.1.2
+      uses: actions/setup-node@b39b52d1213e96004bfcb1c61a8a6fa8ab84f3e8 # v4.0.1
       with:
         node-version: ${{ matrix.NODE_VERSION }}