Years ago, I had Zimbra on Debian, then they dropped support for it. I was able to get my existing installation working on Ubuntu instead. It’s a bit of work doing that kind of, but not too bad and relatively straightforward - certainly much easier than when I moved from Zimbra 32 bit to Zimbra 64 bit. I doubt very much there will be a path to migrate CentOS 7 to Rocky 8 - it didn’t even exist for CentOS (or at least if there was it was rather unstable/unsupported).