Check bundle sizes on Node.js 18 only

The bundles should be identical between Node.js versions, so check with
one version only.

.github/workflows/ci.yaml

@@ -60,19 +60,12 @@ jobs:
       - run: npm run lint
   bundlesize:
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        node: [14, 16, 18]
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3
         with:
-          node-version: ${{ matrix.node }}
+          node-version: 18
       - run: npm ci
-      # Node.js 14 comes with NPM 6 which auto-installs packages by default.
-      - if: ${{ matrix.node == 14 }}
-        run: npx bundlesize
-      - if: ${{ matrix.node != 14 }}
         run: npx --yes bundlesize
   publish:
     if: ${{ github.ref == 'refs/heads/release' }}