Update backend to latest version & Improve documentation

[Mars190]

Disclaimer

Some important things changed, that could possibly break the deployment process:

• .env no longer contains a database url, it now contains the individual variables for it to be crafted by the backend
• The SESSION_SECRET is now required to be atleast 32 characters long, instead of 12
• Docker Compose now also usese these variables to spin up a database with given values
• Updated to yarn 4.7 for the backend
• Changed the name of some commands in package.json

Other changes

• Updated documentation to setup the backend
• All backend dependencies are atleast no longer outdated

[Mars190]

I checked that login and the challenges themselves still work manually, the two tests still pass.