DOC: add instructions to deprecation notice

[ddelange]

• python 3.7 - 3.12 can be supported with numpy v1 build-system
• python 3.9 - 3.13 can be supported with numpy v2 build-system

took me a while to figure this one out, particularly in order to get musllinux_aarch64 working on cp39 in https://github.com/vaexio/vaex/pull/2434

closes #85 and closes #75

[ddelange]

hi @rgommers 👋

taking the liberty to ping you as last committer:)

[ddelange]

green now on my fork: https://github.com/ddelange/oldest-supported-numpy/pull/1