Fix gas network retrofit in brownfield

[tgi-climact]

Changes proposed in this Pull Request

When adding the brownfield for the gas network, the index mismatch leads to 0 capacity of already retrofitted gas pipelines. This results in a remaining capacity that remains constant, while pipelines are being retrofitted. To avoid multiple calculation of the gas network, I also remove the code from the loop.

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.