Switch GitHub Actions to an LTS version of Node (15 -> 16) (#2892)

This should clear up a bunch of version support warnings in
dependencies. Also, in another PR I noticed that `npm ci` for some
reason didn't catch when package-lock.json wasn't updated. Hoping that
this will also fix that.

.github/workflows/pr.yml

@@ -16,7 +16,7 @@ jobs:
           git checkout ${{ github.event.pull_request.head.sha }}
       - uses: actions/setup-node@v2-beta
         with:
-          node-version: "15.x"
+          node-version: "16.x"
       - run: npm ci
       - run: npm test
       - run: |

.github/workflows/push.yml

@@ -13,7 +13,7 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-node@v2-beta
         with:
-          node-version: "15.x"
+          node-version: "16.x"
       - run: npm ci
       - run: |
           npm test

.github/workflows/wpt.yml

@@ -14,7 +14,7 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-node@v3
         with:
-          node-version: "15.x"
+          node-version: "16.x"
       - run: npm ci
       - run: npm run wpt
       - name: copy out-wpt to wpt tree