Bump mysql2 from 3.9.1 to 3.9.4 in /backend

Bumps [mysql2](https://github.com/sidorares/node-mysql2) from 3.9.1 to 3.9.4.
- [Release notes](https://github.com/sidorares/node-mysql2/releases)
- [Changelog](https://github.com/sidorares/node-mysql2/blob/master/Changelog.md)
- [Commits](https://github.com/sidorares/node-mysql2/compare/v3.9.1...v3.9.4)

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

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

backend/package.json

@@ -47,7 +47,7 @@
     "crypto-js": "~4.2.0",
     "express": "~4.19.2",
     "maxmind": "~4.3.11",
-    "mysql2": "~3.9.1",
+    "mysql2": "~3.9.4",
     "rust-gbt": "file:./rust-gbt",
     "redis": "^4.6.6",
     "socks-proxy-agent": "~7.0.0",