Bump webpack-dev-middleware and @microsoft/sp-build-web

Removes [webpack-dev-middleware](https://github.com/webpack/webpack-dev-middleware). It's no longer used after updating ancestor dependency [@microsoft/sp-build-web](http://aka.ms/spfx). These dependencies need to be updated together.


Removes `webpack-dev-middleware`

Updates `@microsoft/sp-build-web` from 1.15.2 to 1.20.1

---
updated-dependencies:
- dependency-name: webpack-dev-middleware
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

samples/react-pnp-js-sample/package.json

@@ -22,7 +22,7 @@
   },
   "devDependencies": {
     "@microsoft/rush-stack-compiler-4.5": "^0.2.2",
-    "@microsoft/sp-build-web": "1.15.2",
+    "@microsoft/sp-build-web": "1.20.1",
     "@microsoft/sp-module-interfaces": "1.15.2",
     "@types/react": "16.9.51",
     "@types/react-dom": "16.9.8",