chore: upgrade react-dom from 16.8.6 to 16.9.0 (#572)

Snyk have raised this PR to upgrade react-dom from 16.8.6 to 16.9.0.

See this package in NPM:
https://www.npmjs.com/package/react-dom

See this project in Snyk:
https://app.snyk.io/org/cds-snc/project/762c54e6-bad5-4b75-b4e7-c2ac53716139?utm_source=github&utm_medium=upgrade-pr

Author: snyk-bot

frontend/package-lock.json

@@ -53,7 +53,7 @@
     "react": "^16.9.0",
     "react-apollo": "^2.5.8",
     "react-datepicker": "^2.8.0",
-    "react-dom": "^16.8.6",
+    "react-dom": "^16.9.0",
     "react-final-form": "^6.3.0",
     "react-ga": "^2.6.0",
     "react-helmet-async": "^1.0.2",