update electricity transmission and offshore connection costs

[fneum]

Closes #167.
Clses #125.

Changes proposed in this Pull Request

• Update electricity transmission and offshore wind connection costs based on latest DEA data (https://ens.dk/en/analyses-and-statistics/technology-data-transport-energy, July 2025).
  Align unit costs of offshore wind connection and general transmission. For offshore wind connections, include missing FOM.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Data source for new technologies is clearly stated.
• Newly introduced dependencies are added to environment.yaml (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the GPLv3 license.