Pypsa v1.1 compatibility and secant-based transmission losses

[lindnemi]

This is a hotfix for pypsa 1.1.

overnight_cost is now a reserved attribute of pypsa. The syntax is subtly different from how it is used here. That's why, for now, I just renamed overnight_cost to onight_cost. When pypsa-eur fully adopts the new overnight_cost, capital_cost, fom_cost syntax, we will deprecate onight_cost and follow pypsa-eur.

Additionally, this PR switches to the new, secant-based transsmission loss approximation available in pypsa1.1.

Before asking for a review for this PR make sure to complete the following checklist:

• Workflow with target rule ariadne_all completes without errors
• The logic of export_ariadne_variables has been adapted to the changes
• One or several figures that validate the changes in the PR have been posted as a comment
• A brief description of the changes has been added to Changelog.md
• The latest main has been merged into the PR
• The config has a new prefix of the format YYYYMMDDdescriptive_title