Bump flat and json2csv

Removes [flat](https://github.com/hughsk/flat). It's no longer used after updating ancestor dependency [json2csv](https://github.com/zemirco/json2csv). These dependencies need to be updated together.


Removes `flat`

Updates `json2csv` from 3.11.5 to 6.0.0-alpha.2
- [Release notes](https://github.com/zemirco/json2csv/releases)
- [Changelog](https://github.com/zemirco/json2csv/blob/master/CHANGELOG.md)
- [Commits](zemirco/json2csv@v3.11.5...v6.0.0-alpha.2)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: json2csv
  dependency-type: direct:production
...

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

Author: dependabot[bot]

package-lock.json

@@ -23,7 +23,7 @@
 		"express-fileupload": "^0.4.0",
 		"express-rate-limit": "^2.11.0",
 		"fast-csv": "^2.4.0",
-		"json2csv": "^3.11.1",
+		"json2csv": "^6.0.0-alpha.2",
 		"mongodb": "^3.0.3",
 		"mongoose": "^5.13.20"
 	},