In most cases, minor OS upgrades are not problematic provided you have
sufficient capacity in your cluster so that it can tolerate scheduled
downtime while some nodes are being upgraded.
One thing you should be aware of are patches in newer versions of Linux
distributions which address Spectre and
OS upgrades most often don't have any impact on C*, and you'd likely only
expect trouble on a major upgrade, but this would typically be related to
package installation, rather than any effects on the database itself.
I wouldn't be too worried about going from RHEL7.4 to 7.9, however going to
8 you
