Fix shakapacker version mismatch: sync package.json with Gemfile.lock (8.0.0)

During rebase, package.json stayed at 8.1.0 while Gemfile.lock was updated to 8.0.0 from master.
This caused CI failure due to shakapacker version checker.

Author: ihabadham

spec/dummy/package.json

@@ -63,7 +63,7 @@
     "sass": "^1.43.4",
     "sass-loader": "^12.3.0",
     "sass-resources-loader": "^2.0.0",
-    "shakapacker": "8.1.0",
+    "shakapacker": "8.0.0",
     "style-loader": "^3.3.1",
     "tailwindcss": "^3.2.7",
     "terser-webpack-plugin": "5",