Promote `{rocky, almalinux, oracle, centos-stream}-9` from experimental

[AkihiroSuda]

EL8 distros do not work on M1:

• https://github.com/lima-vm/lima/issues/841

So I'm planning to promote {rocky, almalinux, oracle, centos-stream}-9 from experimental: e.g., experimental/rocky-9.yaml -> rocky.yaml

The old rocky.yaml will be renamed to rocky-8.yaml, or deprecated/rocky-8.yaml if we want to deprecate EL8 now.