stop using CentOS 7.9 container in CI, no longer works now GitHub Actions is using node.js 20

[boegel]

required because of problems with actions/checkout action like:

Run actions/checkout@v3
/usr/bin/docker exec  9843bae9dc7921cd7e422ca131698a5a550184ebb22dc5829be136c5d40196bd sh -c "cat /etc/*release | grep ^ID"
/__e/node20/bin/node: /lib64/libm.so.6: version `GLIBC_2.27' not found (required by /__e/node20/bin/node)
/__e/node20/bin/node: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.20' not found (required by /__e/node20/bin/node)
/__e/node20/bin/node: /lib64/libstdc++.so.6: version `CXXABI_1.3.9' not found (required by /__e/node20/bin/node)
/__e/node20/bin/node: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by /__e/node20/bin/node)
/__e/node20/bin/node: /lib64/libc.so.6: version `GLIBC_2.28' not found (required by /__e/node20/bin/node)
/__e/node20/bin/node: /lib64/libc.so.6: version `GLIBC_2.25' not found (required by /__e/node20/bin/node)

see also actions/checkout#1809 + https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default

[Micket]

lgtm

[Flamefire]

• CI: Allow using Node 16 actions #4574

[jfgrimm]

looks like we can keep using centos 7.9 for a while longer, by setting an env var (#4574)

[jfgrimm]

I've merged #4574 for now
Once support for node16 is fully dropped, we can revisit dropping the CentOS 7.9 container from the CI

[Flamefire]

IIUC the container is what matters and as they likely won't change the existing container it might never be dropped for this. Let's see.