Updates to support numpy 2

[gelzinyte]

Hi!

I'd like to lift the hard numpy<2 cap, so that Workflow can be used with numpy2-only packages (e.g., fhi-vibes).

Small updates:

• Three calls switched to use numpy-2 syntax
• Changed project dependencies
• Switched the GFN2-xTB backend from the xtb-python package (legacy; restricts to numpy<2) to tblite

Once this PR is merged, would it also be possible to issue a new PyPi release?

[bernstei]

Sounds good. We just updated to numpy 2 internally here as well. I'll review ASAP, but I'm not the one who owns the pypi. We can check the github automated actions - I suspect that if we tag with a new version it'll update on pypi automatically.

[hheenen]

I own the pypi - but I do believe it should just be automatic if you tag the new version. I'll follow here if there are problems.