Bump mysql2 from 2.3.3 to 3.9.7 in /backend/app

Bumps [mysql2](https://github.com/sidorares/node-mysql2) from 2.3.3 to 3.9.7.
- [Release notes](https://github.com/sidorares/node-mysql2/releases)
- [Changelog](https://github.com/sidorares/node-mysql2/blob/master/Changelog.md)
- [Commits](sidorares/node-mysql2@v2.3.3...v3.9.7)

---
updated-dependencies:
- dependency-name: mysql2
  dependency-type: direct:production
...

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

backend/app/package.json

@@ -44,7 +44,7 @@
     "cookie-parser": "^1.4.6",
     "express": "^4.18.2",
     "joi": "^17.7.0",
-    "mysql2": "^2.3.3",
+    "mysql2": "^3.9.7",
     "node-cron": "^3.0.2",
     "otplib": "^12.0.1",
     "passport": "^0.6.0",