Bump react-dom from 16.2.0 to 16.2.1 in /SlackNotifications/ClientApp

Bumps [react-dom](https://github.com/facebook/react/tree/HEAD/packages/react-dom) from 16.2.0 to 16.2.1.
- [Release notes](https://github.com/facebook/react/releases)
- [Changelog](https://github.com/facebook/react/blob/master/CHANGELOG.md)
- [Commits](https://github.com/facebook/react/commits/HEAD/packages/react-dom)

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

Author: dependabot[bot]

SlackNotifications/ClientApp/package-lock.json

@@ -6,7 +6,7 @@
     "bootstrap": "^4.3.1",
     "jquery": ">=3.5.0",
     "react": "^16.0.0",
-    "react-dom": "^16.0.0",
+    "react-dom": "^16.2.1",
     "react-router-bootstrap": "^0.24.4",
     "react-router-dom": "^4.2.2",
     "react-scripts": "^1.1.5",