Update pixi to 0.21.1

[deltamarnix]

Explanation

Update pixi to the latest version so people with multiple Deltares repositories on their PC don't have to switch pixi versions.

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Tests & pre-commit hooks pass
• Updated documentation if needed
• Updated changelog.rst if needed
• For predefined catalogs: update the catalog version in the file itself, the references in data/predefined_catalogs.yml, and the changelog in data/changelog.rst

Additional Notes (optional)

Add any additional notes or information that may be helpful.

[savente93]

LGTM we'll have to try and take advantage of the new features in a seperate PR