Yes, we had to increase the minimum supported SkiaSharp due to vulnerabilities in 2.88 (see for example Vulnerable package SkiaSharp 2.88.0 is used. )
But that was the only reason we raised the requirement, FlexCel 7.23 should still work with 2.88 if you want (of course, please don't use 2.88.0 or any other vulnerable version, but 2.88.9 should work). You would need to recompile FlexCel dll requiring 2.88.9, but it is doable.
I can't sadly help much more here, since this is a skia sharp issue, they should fix it. There is not much we can do here