Documentation improvement for support OS version and etcd backup

[2ez4szliu]

Issue #, if available:

Description of changes:

1. Removing the line that says RHEL 9 is only supported on baremetal, nutanix and cloudstack
2. Call out that the etcd backup process in our doc is for external etcd and baremetal provider only supports stacked etcd.

Testing (if applicable):

Documentation added/planned (if applicable):

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