Finish GraphiQL 6 examples and release references

[trevor-scheer]

This is the final cleanup PR for graphiql-6 before the stable majors become npm latest and the branch merges to main. The main CDN example now uses the published GraphiQL 6 RC packages, transport, and the v6 default plugins; the code-exporter example still needs its release-time update.

The CDN example was regenerated with node .github/scripts/update-cdn-versions.mjs --graphiql-tag rc. The script defaults to latest and accepts an optional tag for the three GraphiQL packages. Its worker import pins @graphiql/react to the selected version because esm.sh currently returns 404 for the RC range in the unpinned helper. This also fixes the migration-guide link and documents how to serve and regenerate the example. The stable packages aren't published yet, so this draft still needs the release-time changes below before it can merge:

• Remove the RC-only query builder changesets from Return query builder to operation after fragment deletion #4650 and Follow typed operations in the query builder #4651 after publishing @graphiql/plugin-query-builder@1.0.0-rc.3, leaving their RC changelog entries in place.
• Exit Changesets prerelease mode; apply and review stable package versions, internal ranges, changelogs, and the lockfile.
• Remove GraphiQL-family beta/rc references from active package manifests and example dependencies. Leave unrelated upstream prereleases and historical changelogs alone.
• Point the esm.sh GraphQL worker helper and declaration at the stable monaco-graphql major.
• Regenerate examples/graphiql-cdn/index.html without --graphiql-tag rc to switch back to latest before merge, update its README, and update packages/graphiql-plugin-code-exporter/example/index.html to the stable v6 package family. Serve each page and check queries, completion, diagnostics, formatting, shortcuts, all Monaco workers, and code export.
• Remove the graphiql-6 release-workflow trigger after the last RC publish.

The scheduled CDN updater continues to use latest; RC generation is opt-in.

Supersedes #4587. Refs #4219.

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