Bump phoenix from 1.7.20 to 1.7.21 in /app/assets

Bumps [phoenix](https://github.com/phoenixframework/phoenix) from 1.7.20 to 1.7.21.
- [Release notes](https://github.com/phoenixframework/phoenix/releases)
- [Changelog](https://github.com/phoenixframework/phoenix/blob/main/CHANGELOG.md)
- [Commits](https://github.com/phoenixframework/phoenix/commits)

---
updated-dependencies:
- dependency-name: phoenix
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

app/assets/package-lock.json

@@ -54,7 +54,7 @@
     "js-cookie": "^3.0.5",
     "moment": "^2.30.1",
     "node-webvtt": "^1.9.4",
-    "phoenix": "^1.7.20",
+    "phoenix": "^1.7.21",
     "prop-types": "^15.8.1",
     "rc-progress": "^4.0.0",
     "react-beautiful-dnd": "^13.1.0",