Merge pull request rails#40909 from yahonda/lock_webpacker_version_to_5

Lock rails/webpacker npm package version to 5

Author: kamipo

railties/test/isolation/assets/package.json

@@ -5,7 +5,7 @@
     "@rails/actioncable": "file:../../../../actioncable",
     "@rails/activestorage": "file:../../../../activestorage",
     "@rails/ujs": "file:../../../../actionview",
-    "@rails/webpacker": "https://github.com/rails/webpacker.git",
+    "@rails/webpacker": "^5.2.1",
     "turbolinks": "^5.2.0"
   }
 }