🚀 [Feature] Upgrade to node 15.4

[Mogge]

🚀 Feature

It would be nice to have our code running on the latest node version (15.4 by now), which is not the case.
Known problems:

• 🐛 [Bug] Image upload is not working in local development environment #4081
• 💥 [DevOps] node-sass is deprecated #4068 (fixed)

Until these issues are not fixed. There is no way to update our node version.
May be there are more issues which have not been discovered yet.

Todo

• Please make sure to update the backend/README.md when we upgrade our node version.
• And mention Node Version Manager nvm for the parallel installation of different node versions and thier usage in different folder/Terminals.
• .nvmrc files in the app folders can be helpful to quickly use the right Node version in a folder/terminal.

[Mogge]

Please make sure to update the backend/README.md when we upgrade our node version.