Kernel update from 4.19 to 5.10

Hello,
I want to update my own image based on the following KUNBUS image 2019-03-14-revpi-stretch in order to be able to use it on the CM4S.
I loaded the image on a CM3+ (Article No.: 100304) and I tried the procedure described on the tutorial "Migration to CM4S based devices" https://revolutionpi.com/tutorials/migration-cm4s/
I followed point 2 of the tutorial and after the update procedure I was able to update the kernel up to version 4.19.95 as it tells me it is the newest version and no further versions are available.
What is the reason why I cannot install the newest 5.10 version of the kernel?

Is there a special reason for using the old stretch image? If not I would strongly suggest to use Buster, which has out of the box support for the core s (since 2022-08).

Nicolai

This Stretch image include some software and code that were developed to fulfill the requirements of a specific application.
As I do not have the know how to rebuild the full application starting from Buster then I think the only way is to update the kernel.
In the migration document the Stretch image is listed as upgradable to kernel 5.10, isn't it?