test: add rhel-8-4 and rhel-atomic to README.md
test: add rhel-8-4 and rhel-atomic to README.md

These are supported platforms to run the tests on.

Closes #14910
garrett/cockpit,martinpitt/cockpit,garrett/cockpit,cockpit-project/cockpit,deryni/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,mvollmer/cockpit,deryni/cockpit,cockpit-project/cockpit,garrett/cockpit,martinpitt/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit