Mention removing overrides as initial step in upgrading from v5 to v6

[mcoker]

I'm not sure if we call this out as a good first step in the upgrade process anywhere, but I found this on https://staging.patternfly.org/get-started/upgrade/#2.-review-and-update-variable-and-class-names

As much as possible, we recommend removing your CSS overrides so that your product upgrade experience will be smoother for future releases.

I'm wondering if it might also be good to mention on that page and/or on https://staging.patternfly.org/tokens/develop-with-tokens#migrate-to-tokens that a great first step in updating v5 style overrides would be to remove all of your overrides and see how things look, and add back any necessary customizations since there often isn't a 1:1 equivalent of a v5 style to a v6 style, and overrides from v5 may not even be relevant in v6. To quote Joachim during the initial POC to update OCM to v6:

I have a feeling it may almost be better to exclude all custom PF overrides to begin with, then add in as needed. I experimentally null loaded all our [style overrides] and it’s not too bad

cc @srambach @edonehoo @nicolethoen