Only add buses in specified countries

[koen-vg]

This is necessary for the new OSM-based base network, where loaded buses already have a "country" attribute.

Closes https://github.com/PyPSA/pypsa-eur/issues/1306

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.

[koen-vg]

If I understand the intention of dropping non-OSM support correctly, the if-statement checking if "countries" is a column in buses can be simplified once OSM is the only source. I could add a note to that effect, but it seems to me like a lot of code can be simplified at that point, so hopefully that should be obvious at that point?