DOC, TST: make numpy.version officially public

[rgommers]

Also update the public API tests for the actual state of 2.0.x/main

Closes gh-26072

[rgommers]

Note: we discussed making numpy.version officially public in the community meeting on Wednesday, and everyone was happy / in favor. There's a bunch of downstream usage of numpy.version, so in practice this changes very little. The window for somewhat disruptive 2.0 API cleanups is closed, so this is kinda the only reasonable option at this point.

[charris]

Thanks Ralf.