ci: drop testing on CentOS 7 in packaging workflow

It's impossible to run Node.js 20 actions on CentOS 7 due to issue [1].
So if we want to upgrade to Node.js 20 actions, we have to drop testing
on CentOS 7 in the packaging workflow. Otherwise, the testing job fails
with the following error:

    /__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)

[1] https://github.com/actions/checkout/issues/1487

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci