fix: reverting the markdown bump in #1044

[erunion]

🧰 What's being changed?

• Reverting fix(markdown): pinned callouts not rendering body #1044. There's some issues with Markdown callouts in production.
• Pinning our @readme/syntax-highlighter dependency to 10.2.*.
  • This was done because there's a bug in 10.3 where the tomorrow-night theme was removed. We're working on a fix to this in feat: request body raw/json mode editor #1050, but until there's some tests in next that are broken because NPM decides to upgrade us to the 10.3 series when you run npm ci.

I'm moving @readme/markdown into being a peerDep and Webpack external and that should hopefully fix these issues from cropping up.