Add missing OCGT discount rates

[danielelerede-oet]

Changes proposed in this Pull Request

This PR adds technology-specific discount rates for OCGT in US data as originally missing (NREL ATB doesn't specifically include a technology to be associated with OCGT, but discount rates for gas plants are all the same).

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.