Bump @types/shelljs from 0.8.12 to 0.8.15 in /backend

Bumps [@types/shelljs](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/shelljs) from 0.8.12 to 0.8.15.
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/shelljs)

---
updated-dependencies:
- dependency-name: "@types/shelljs"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

backend/package.json

@@ -95,7 +95,7 @@
     "@types/node": "^20.10.1",
     "@types/nodemailer": "^6.4.14",
     "@types/pako": "^2.0.0",
-    "@types/shelljs": "^0.8.12",
+    "@types/shelljs": "^0.8.15",
     "@types/supertest": "^2.0.16",
     "@types/unzipper": "^0.10.7",
     "@types/uuid": "^9.0.1",