Improve solar and wind modeling

[lindnemi]

This PR does several things:

• The correction_factor for solar introduced in Solar Correction Factor #225 is adjusted downwards to 0.9. The reason was a higher capacity factor observed for solar in the latest 3H and 30 nodes (for DE) runs. Besides, in hindsight, it seems overengineered to pick a very precise number for something that is not know that exactly, and that we don't expect to be exact in our model. Afterall we are mixing 2020 capacities with a 2019 weather year and extrapolate that into the future. So, 0.9 is a nice number, does the job, and was stated by Tom as the maximum acceptable correction factor before the linear downscale would distort peak capacity (and thus required Netzausbau) too much.
• For similar reasons a correction_factor of 0.95 was introduced for offwind. We are orienting towards the follwing data:

• We take over an expected delay in one of the offshore projects (closes Verzögerungen bei Offshore Projekten #283).

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