This CKEditor 4.22.1

Thank you for bringing this to my attention.

Unfortunately, the latest (and last) open-source version of CKEditor 4 (CKEditor 4.22.1, the version used within my unofficial update patch for Showkase v1.7.6) is no longer receiving updates and the long-term support versions of CKEditor 4 (which should be more secure) require a license for commercial use (which we don't currently have).
It looks like CKEditor 5 would also requires a commercial license for use within Showkase (being free only for open-source projects).

If you are willing to accept the security risks, then you can at least suppress the warning by adding:

config.versionCheck = false;

... to Showkase's showkase\admin\settings\ckeditorconfig.js file.

I don't know exactly what the security risks are (I have not yet been able to find details on CKEditor's website) or, indeed, which versions of CKEditor they affect.
It might actually be beneficial to revert to CKEditor 4.16 (from the Showkase v1.7.6 zip package). If you'd like to do this, just overwrite the showkase\admin\ckeditor folder on your web server with the showkase\admin\ckeditor folder from the Showkase v1.7.6 zip package. I genuinely don't know if this will help (with the security itself or the warning message) but it might be something you'd like to try.
As always, when making changes manually to files on your web server, I'd recommend making a full backup of your site first, just as a precaution.

It looks like we might need to stick with CKEditor 4.16 (from the official Showkase v1.7.6 zip package) or CKEditor 4.22.1 (from my patch) for the time being until the developers can formulate a more long-term solution.

Maybe there's a fork of the open-source CKEditor 4.22.1 somewhere on GitHub that might be a suitable replacement? I'll take a look.
I'll post back here if I have any further news on the subject.