Support pandas >= 3

[timmens]

Problem

gradient-free-optimizers requires pandas<3.0.0 (even in its latest release, 1.10.1). This forces us to pin pandas<3 in our pixi configuration for all environments.

See: https://pypi.org/project/gradient-free-optimizers/1.10.1/

Desired behavior

Once gradient-free-optimizers releases a version compatible with pandas 3, we should:

1. Remove the pandas<3 pin from pyproject.toml ([tool.pixi.dependencies])
2. Verify all tests still pass

Alternatively, if gradient-free-optimizers is unlikely to update, we could consider isolating gfo into its own environment with the pandas pin, allowing the rest of the project to use pandas 3+.

Context

This was identified during the conda-to-pixi migration (#674).