Mention css-vars-updater codemod package in the upgrade guide

[adamviktora]

A new codemod package css-vars-updater has been released.

We should mention this in the https://www.patternfly.org/get-started/upgrade site.

Currently, the tokens-update codemod is mentioned:

The css-vars-updater package updates CSS variables in all kinds of files (css, scss, md, or whatever extension the user specifies), not only React code.
However the tokens-update codemod is still preferable for updating React code (it can also update react tokens), this new package is meant for updating just css variables in .css and other file extensions, where the tokens-update doesn't work.