Bump celebrate from 12.2.0 to 15.0.3 in /backend

Bumps [celebrate](https://github.com/arb/celebrate) from 12.2.0 to 15.0.3.
- [Release notes](https://github.com/arb/celebrate/releases)
- [Commits](arb/celebrate@v12.2.0...v15.0.3)

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

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

Author: dependabot[bot]

backend/package-lock.json

@@ -32,7 +32,7 @@
         "bcrypt": "^5.0.0",
         "bluebird": "^3.7.2",
         "body-parser": "^1.19.0",
-        "celebrate": "^12.1.1",
+        "celebrate": "^15.0.3",
         "cloudinary": "^1.21.0",
         "cron": "^1.8.2",
         "dataloader": "^2.0.0",