Bump lodash from 4.17.15 to 4.17.21 in /functions

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.15 to 4.17.21.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](https://github.com/lodash/lodash/compare/4.17.15...4.17.21)

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

README.md

@@ -1,4 +1,4 @@
 [![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/estevez-dev/ha_client_fcf) 
 
-## Firebase Cloud Functions for [HA Client app](https://github.com/estevez-dev/ha_client)
+## Firebase Cloud Functions for [HA Client app](https://ha-client.app)
 Based on [Push Notification Sender for Home Assistant official mobile apps](https://github.com/home-assistant/mobile-apps-fcm-push).

functions/package-lock.json

@@ -1453,9 +1453,9 @@
       }
     },
     "lodash": {
-      "version": "4.17.15",
-      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.15.tgz",
-      "integrity": "sha512-8xOcRHvCjnocdS5cpwXQXVzmmh5e5+saE2QGoeQmbKmRS6J3VQppPOIt0MnmE+4xlZoumy0GPG0D0MVIQbNA1A=="
+      "version": "4.17.21",
+      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.21.tgz",
+      "integrity": "sha512-v2kDEe57lecTulaDIuNTPy3Ry4gLGJ6Z1O3vE1krgXZNrsQ+LFTGHVxVjcXPs17LhbZVGedAJv8XZ1tvj5FvSg=="
     },
     "lodash.at": {
       "version": "4.6.0",