♻️ Simplify env files, merge to one .env.

Having multiple files for env variables could help to identify which configs belong to which services, but the same can be done with comments.

A single .env file could help to have a general idea of all the configs in a single place.
It will also help to simplify the development of the project generator itself, as env var templates now live in a single file that can be generated more easily.

Having files separated could help if there were conflicts between env var names for different services, but that's not the case. The other reason would be to not expose env vars to some services if they were not trusted, but that's also not the case here, all the services in this stack are part of the app itself and "trusted", so they can get the env vars, they just won't use them.