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karut In case you missed my message above, Ubuntu 18.04 gets security updates until 2028 through Ubuntu Pro ESM. AlmaLinux gets security updates until 2029.
With Ubuntu you have the option of using kernel 4.15 and with AlmaLinux (binary-compatible with RHEL) you have the option of using kernel 4.18.
I think these are the two oldest kernels that will receive security updates for the next 4-5 years.