feat: add react breaking changes list to release notes

[nicolethoen]

Closes #3907

view it running on: https://patternfly-org-pr-4259-site.surge.sh/get-started/upgrade/release-notes

[patternfly-build]

Preview: https://patternfly-org-pr-4259-site.surge.sh

[edonehoo]

It looks like some of the formatting isn't applying, like the backtick code styling (maybe not too big a deal) and markdown tables (matters more)

[nicolethoen]

I'll take another pass through and clean up bcakticks and markdown tables. I"d been trying to remove those 👍🏻

[nicolethoen]

I don't think it's super quick to fix the backticks so they are styled like other markdown pages. I think that could be a follow up if needed.

[edonehoo]

I can still see some md table formatting when I view the preview link, but I can also see that you removed it in your pr, so I think it's just the preview showing that. I don't think the backtick/code formatting really matters honestly--especially since it's time consuming to edit--devs will understand it regardless. I can edit in a followup if we do want the visual consistency, though!