Upgrade from centoS 6.10 to what OS for OTRS/Znuny?
Solution 1:
You must make the decision about what distribution to use.
Are you able to deploy your applications on the distro? Do you trust the maintainers to provide timely security updates for the life of the release?
Maybe a dozen RHEL clones exist, per DistroWatch or Wikipedia. Fedora and derivatives if you are fine with moving fast and upgrading every year, but perhaps not if CentOS Stream is too fast for you. Many more if you do not limit yourself to Red Hat. Even more if you are not limited to Linux and can try say one of the BSDs.