Align the naming convention for the CO2 network configuration

[tgilon]

Closes # (if applicable).

Changes proposed in this Pull Request

Align the naming convention for the CO2 network configuration (from co2network to co2_network). This may be a small breaking change.

Checklist

• [x] I tested my contribution locally and it works as intended.
• [ ] Code and workflow changes are sufficiently documented.
• [ ] Changed dependencies are added to envs/environment.yaml.
• [ ] Changes in configuration options are added in config/config.default.yaml.
• [ ] Changes in configuration options are documented in doc/configtables/*.csv.
• [ ] Sources of newly added data are documented in doc/data_sources.rst.
• [x] A release note doc/release_notes.rst is added.