Update prop-types requirement (Graylog2#5841)

Updates the requirements on [prop-types](https://github.com/facebook/prop-types) to permit the latest version.
- [Release notes](https://github.com/facebook/prop-types/releases)
- [Changelog](https://github.com/facebook/prop-types/blob/master/CHANGELOG.md)
- [Commits](facebook/prop-types@v15.6.2...v15.7.2)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-preview[bot]

graylog2-web-interface/packages/graylog-web-plugin/package.json

@@ -42,7 +42,7 @@
     "jquery": "3.4.1",
     "moment": "2.22.2",
     "moment-timezone": "0.5.23",
-    "prop-types": "15.6.2",
+    "prop-types": "15.7.2",
     "react": "16.8.3",
     "react-addons-pure-render-mixin": "15.6.2",
     "react-bootstrap": "0.31.5",