Follow scientific python ecosystem guidelines for dependencies

[agoscinski]

@giovannipizzi mentioned to check if we align with official numpy recommandation guidelines for software dependencies as menitoned in AEP 003, referencing NEP 0029 which was superseeded by SPEC 0000. Anyway topic for another PR

Originally posted by @agoscinski in #6899 (comment)

[danielhollas]

One obvious thing to do here is to drop support for Python 3.9, which will become officially unsupported soon anyway (in October this year I believe). We now support 5 (!) python versions, and 3.14 is going to be released soon as well. Happy to work on this during the coding days if people agree.

The only wrinkle here is that we're still stuck on Python 3.9 in AiiDAlab docker image. But we should really get our sh*rt together and upgrade. I've spend a fair amount of time on that already, somebody just needs to push it over the finish line, see e.g. aiidalab/aiidalab-docker-stack#455.

Since aiida 2.7 has just been released, I am fine for 2.8 to drop support, which should give us enough time to upgrade (assuming some resources are dedicated to that).