Relax numpy dependency to <=2.1.0

[tcompa]

Some time ago we relaxed a numpy version constraint from "~1.23.5" to <2 (https://github.com/fractal-analytics-platform/fractal-tasks-core/pull/477/files). We did not mention any specific reason for not supporting v2, but maybe it was not even released yet.

We should test whether we can support numpy v2 (I'd say yes, by looking at the release notes), and relax the version constraint to accept v2.

Note that starting from 2.1.0 Python 3.9 is not supported any more. This is not an issue for us, if we use a constraint like numpy<=2.1.0 (which will install 2.0.2 on Python 3.9 and 2.1.0 on more recent Python).