CI: Test NumPy build against old versions of GCC(6, 7, 8)

[seiko2plus]

closes #21920

[charris]

@rgommers What is the oldest gcc SciPy is going to support?

[rgommers]

@rgommers What is the oldest gcc SciPy is going to support?

We bumped it from GCC 4.9 to GCC 8 for the next release: https://github.com/scipy/scipy/blob/0fc5126bf0cfd7d0f46c845003e0c52094d4019b/meson.build#L27-L37

The change in this PR is consistent with what we discussed in the developer meeting. From the meeting notes: GCC versions: we need to test with a wider range of compilers, and drop older compilers. Sayed would like GCC 9, that’s stable for AVX512/SkylakeX features. Also influenced by C++ code. We need C99 and C++11. Proposal: minimum version gcc 6.3 and error out if the compiler is older.

I think GCC 8 would be fine, but doing 6.3 now and waiting if there's major complaints for SciPy before following suit is also fine.

[rgommers]

but doing 6.3 now and waiting if there's major complaints for SciPy before following suit is also fine.

Given that what we get from Ubuntu is 6.5, let's make that the documented minimum version. I added a commit that updates the docs in two places to state this.

[rgommers]

LGTM, thanks @seiko2plus!

This should be backported to 1.23.3, and it would be good to add it to those release notes.

[charris]

Thanks @seiko2plus .