Bump mobx from 6.3.6 to 6.4.2 in /website

Bumps [mobx](https://github.com/mobxjs/mobx) from 6.3.6 to 6.4.2.
- [Release notes](https://github.com/mobxjs/mobx/releases)
- [Commits](https://github.com/mobxjs/mobx/compare/mobx@6.3.6...mobx@6.4.2)

---
updated-dependencies:
- dependency-name: mobx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

website/package.json

@@ -14,7 +14,7 @@
     "common-tags": "^1.8.2",
     "date-fns": "^2.28.0",
     "google-protobuf": "^4.0.0-rc.1",
-    "mobx": "^6.3.6",
+    "mobx": "^6.4.2",
     "mobx-react": "^7.2.1",
     "mobx-utils": "^6.0.4",
     "numeral": "^2.0.6",