Conda: update YAML for the main environment

Update the main conda YAML file, i.e. snakemake and other potentially relevant packages.

This should be the default conda environment to be used by all developers and to be recommended to the user as it will be used for testing as well.

Suggested version: 6.7.0, used together with mamba.

• update requirements.yaml
• move requirements.yaml out of config/
• mention in the documentation (i.e. installation instructions)