Replace CentOS 7 with CentOS Stream 8 in CI
CentOS 7 has been broken in CI for many months, and there
are no deployments with the latest code on CentOS 7 anymore.
This also begins the end of Python 2 support, as we are no longer
testing on Python 2 at all.
Signed-off-by: Neal Gompa <ngompa13@gmail.com>