Bump xml2js and @microsoft/sp-build-web in /samples/react-app-settings

Bumps [xml2js](https://github.com/Leonidas-from-XIV/node-xml2js) to 0.5.0 and updates ancestor dependency [@microsoft/sp-build-web](http://aka.ms/spfx). These dependencies need to be updated together.


Updates `xml2js` from 0.4.23 to 0.5.0
- [Commits](https://github.com/Leonidas-from-XIV/node-xml2js/commits/0.5.0)

Updates `@microsoft/sp-build-web` from 1.15.2 to 1.17.2

---
updated-dependencies:
- dependency-name: xml2js
  dependency-type: indirect
- dependency-name: "@microsoft/sp-build-web"
  dependency-type: direct:development
...

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

samples/react-app-settings/package.json

@@ -21,7 +21,7 @@
     "react-dom": "16.3.2"
   },
   "devDependencies": {
-    "@microsoft/sp-build-web": "1.15.2",
+    "@microsoft/sp-build-web": "1.17.2",
     "@microsoft/sp-module-interfaces": "1.7.1",
     "@microsoft/sp-webpart-workbench": "1.12.1",
     "gulp": "~3.9.1",