Move dotenv dependency to deps rather than devDeps
Move dotenv dependency to deps rather than devDeps

The dotenv package is being included in the main `server.js` file
and is therefore required in production too.

uktrade/data-hub-frontend,uktrade/data-hub-frontend,uktrade/data-hub-fe-beta2,uktrade/data-hub-frontend,uktrade/data-hub-fe-beta2