Add additional steps running as non-root on old Kernels, e.g. RHEL7

When installing on very old Kernel versions .e.g on RHEL 7 ~3.10 the function to open datagram sockets as an unprivileged user doesn't exist. We need to document a workaround adding CAP_NET_RAW capabilities to the process to run as non-root user. We have documented a way to solve this problem in a knowledgebase article: https://opennms.discourse.group/t/h29-wont-start-with-permission-error-to-open-icmp-socket/2387

It would be helpful to add this in our installation section mostly for RHEL 7 users with additional steps to set the capabilities in the systemd unit.