Bump firebase from 7.14.0 to 9.15.0 in /example

Bumps [firebase](https://github.com/firebase/firebase-js-sdk) from 7.14.0 to 9.15.0.
- [Release notes](https://github.com/firebase/firebase-js-sdk/releases)
- [Changelog](https://github.com/firebase/firebase-js-sdk/blob/master/CHANGELOG.md)
- [Commits](https://github.com/firebase/firebase-js-sdk/compare/firebase@7.14.0...firebase@9.15.0)

---
updated-dependencies:
- dependency-name: firebase
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

example/package.json

@@ -11,7 +11,7 @@
   },
   "dependencies": {
     "@firebase/firestore": "^1.12.0",
-    "firebase": "^7.14.0",
+    "firebase": "^9.15.0",
     "prismjs": "^1.19.0",
     "prop-types": "^15.7.2",
     "react": "^16.13.1",