fix and rename discount rate wildcard

[ljansen-iee]

Closes # (if applicable)

See #1583. The issue could be closed after the merge of this PR

Changes proposed in this Pull Request

• Currently the discountrate wildcard value is ignored in the workflow for elec and sector networks. This PR fixes this by adding snakemake.params.costs["fill_values"]["discount rate"] = snakemake.params.costs["discount_rate"][0] once before load_costs / prepare_costs calls. Like that we insert the discount_rate config value (the wildcard) into fill_values.
• This PR removes a in after that redundant or inconsistent "discount rate" field from the costs.fill_values section in config.default.yaml to avoid confusion.
• Furthermore it standardizes the naming of the discount rate parameter across the codebase, changing all instances of discountrate to discount_rate for consistency and clarity. This affects configuration files, documentation, and all relevant Snakemake rules and file patterns.

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented, including updates to docstrings for meaningful functions.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/user-guide/configuration.md and doc/tutorials/electricity-model.md.
• If config sections were added, renamed, or removed, update doc/assets/scripts/extract_config_snippets.py accordingly.
• Archives of the uploaded data do not have an enclosing folder and archive names correspond to the conventions of configs/bundle_config.yaml.
• A note for the release notes doc/release-notes.md is amended in the format of previous release notes, including reference to the requested PR.