keras3-cpu: Fix broken PDF export of notebooks

To export a Jupyter Notebook as PDF from Jupyter Lab or VS Code
nbconvert is used. nbconvert calls pandoc to generate LaTeX source and
compile it as PDF. nbconvert uses a custom LaTeX template, which is not
compatible with pandoc >= 3.2.1, currently. This commit is a workaround
to make it compatible. Revert this commit as soon as
https://github.com/jupyter/nbconvert/pull/2196 is available in the base
docker image. However, the workaround should not do any harm, since it
checks whether the patch is required before modifying the template.