🐛 fix key_error in validation plots

[Tomkourou]

Closes # (if applicable).

Changes proposed in this Pull Request

Currently, running the validation workflow fails at the plot_validation_electricity_production rule due to a mismatch between historic data keys and the outputs of the pypsa network. The submitted code corrects technology keys for the rule to work out the box.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[fneum]

Thanks @Tomkourou, I don't think we'll maintain the validation plotting in detail and maybe even outsource it from the main PyPSA-Eur repository prospectively.

I'll merge your changes without a detailed review.