Bump serve-static and express in /backend

Bumps [serve-static](https://github.com/expressjs/serve-static) to 1.16.2 and updates ancestor dependency [express](https://github.com/expressjs/express). These dependencies need to be updated together.


Updates `serve-static` from 1.15.0 to 1.16.2
- [Release notes](https://github.com/expressjs/serve-static/releases)
- [Changelog](https://github.com/expressjs/serve-static/blob/v1.16.2/HISTORY.md)
- [Commits](https://github.com/expressjs/serve-static/compare/v1.15.0...v1.16.2)

Updates `express` from 4.19.2 to 4.21.0
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/4.21.0/History.md)
- [Commits](https://github.com/expressjs/express/compare/4.19.2...4.21.0)

---
updated-dependencies:
- dependency-name: serve-static
  dependency-type: indirect
- dependency-name: express
  dependency-type: direct:production
...

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

backend/package.json

@@ -45,7 +45,7 @@
     "axios": "1.7.2",
     "bitcoinjs-lib": "~6.1.3",
     "crypto-js": "~4.2.0",
-    "express": "~4.19.2",
+    "express": "~4.21.0",
     "maxmind": "~4.3.11",
     "mysql2": "~3.11.0",
     "rust-gbt": "file:./rust-gbt",