code: modify target branch for ci triggering

[finozzifa]

Closes # (if applicable).

Changes proposed in this Pull Request

This pull request modifies the .github/ci.yaml such that the CI is triggered by means of actions onto the prototype-2 branch. Formerly, actions were triggered by actions onto the ai-prototype branch.

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.