.github: upgrade to checkout@v4 and setup-python@v5

[marc-hb]

Long overdue; both where still at @v1!

Found by chance in https://github.com/zephyrproject-rtos/west/actions/runs/9275086435/job/25518959121?pr=711 where all macos-latest configs failed like this:

Run actions/setup-python@v1
Error: Version 3.8 with arch x64 not found
Available versions:
  (empty)

[marc-hb]

@mbolivar-ampere , please?

This is now blocking all other PRs like for instance #713