Use npm ci again, but bump npm to v7 as recommended in actions/setup-node#213

Author: davwheat

.github/workflows/pr_size_change.yml

@@ -25,8 +25,11 @@ jobs:
         with:
           node-version: "14"
 
+      - name: Use npm v7
+        run: sudo npm install -g npm@7.x.x
+
       - name: Install JS dependencies
-        run: npm install
+        run: npm ci
         working-directory: ./js
 
       - name: Build production assets