Add back deprecated runners with warning

[paulo-ferraz-oliveira]

Description

This broke for some consumers, but if we state (as a code warning, and by not having it in the README.md) that support is limited, this might not only nudge consumers in the right direction but also "support" those that want to work with deprecated elements.

• I have read and understood the contributing guidelines

[paulo-ferraz-oliveira]

Done for #332 (comment). @petermm, feel free to continue here, especially if you can provide feedback on results (I can't test this on GHA, since ubuntu-20.04 is actually not usable).

[petermm]

awesome!
just one blocker: Error: core.warn is not a function

it's core.warning https://github.com/actions/toolkit/tree/main/packages/core#logging

have CI run(s) ready: https://github.com/petermm/AtomVM/actions/runs/15862621970/job/44723112273

but no hurry.

thanks!

[paulo-ferraz-oliveira]

I've pushed a fix.

[paulo-ferraz-oliveira]

I've pushed a fix.

[paulo-ferraz-oliveira]

Given the limited change scope, if tests pass per @petermm's feedback, I'm merging, to cut 0.20.1.

[petermm]

It's green and works now: https://github.com/petermm/AtomVM/actions/runs/15862621970/job/44725251953#step:8:27

[paulo-ferraz-oliveira]

This was released in v1.20.1.

[petermm]

Thank you for accommodating this obscure usage of old OS/compilers!