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-provision-assets/package.json

@@ -18,7 +18,7 @@
     "@types/react-addons-test-utils": "0.14.15"
   },
   "devDependencies": {
-    "@microsoft/sp-build-web": "~1.15.2",
+    "@microsoft/sp-build-web": "~1.20.1",
     "@microsoft/sp-module-interfaces": "~1.1.0",
     "@microsoft/sp-webpart-workbench": "~1.12.1",
     "gulp": "~4.0.2",