chore(deps): bump mjml from 4.15.0 to 5.0.0 in /backend

Bumps [mjml](https://github.com/mjmlio/mjml/tree/HEAD/packages/mjml) from 4.15.0 to 5.0.0.
- [Release notes](https://github.com/mjmlio/mjml/releases)
- [Commits](https://github.com/mjmlio/mjml/commits/v5.0.0/packages/mjml)

---
updated-dependencies:
- dependency-name: mjml
  dependency-version: 5.0.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/package.json

@@ -42,7 +42,7 @@
     "helmet": "4.6.0",
     "ioredis": "4.28.5",
     "lru-cache": "11.5.1",
-    "mjml": "4.15.0",
+    "mjml": "5.0.0",
     "mongodb": "6.3.0",
     "mustache": "4.2.0",
     "nodemailer": "8.0.11",