Release v3.9 #19332

We have made enough changes in the main branch.

The MDX upgrade still not finished, and it may require a major release, so we won't wait for it.

In this verison we

• Updated the markdown parser to the latest micromark
• Updated the yaml package to v2
• Support GraphQL v17, Will wait for graphql v17 for a few days (v17 Release Target: 2026-06-15 === today! graphql/graphql-js#4786)
• Major changes in js/ts printer

I'm setting the release date to June 20.

//cc @seiyab @kovsu

• prepare
  • Complete milestone items
  • Run on other projects to check for regressions
    Regressions in main branch (v3.9) #19328
  • Write release notes Add blog post for v3.9 #19414
    • use node scripts/draft-blog-post.mjs to generate a blog post draft (a file named YYYY-MM-DD-version.md in website/blog) from changelog_unreleased
    • prs-merged-since (readme) may be useful
• publish
  • Run release script
• postpublish
  • Wait for release notes to deploy
  • Tweet from @PrettierCode account via TweetDeck
  • Close milestone
  • Send schema PR generated by node scripts/generate-schema.mjs to SchemaStore if we added a new option.
  • Update the release checklist with possible changes

Then monitor Twitter/GitHub for any immediate issues 😄