chore: upgrade next-mdx-remote to v6 in apps/docs (#42748)

## I have read the CONTRIBUTING.md file.

YES

## What kind of change does this PR introduce?

Dependency upgrade (next-mdx-remote v4 → v6)

## What is the current behavior?

The docs app uses next-mdx-remote v4.4.1 with MDX v2.

## What is the new behavior?

- Upgraded to next-mdx-remote v6.0.0 (uses MDX v3)
- Updated @mdx-js/loader and @mdx-js/react to v3
- Upgraded remark-gfm to v4 for MDX v3 compatibility
- Removed deprecated `useDynamicImport` option (now default)
- Added `blockJS: false` to preserve JS expressions in MDX content

Build compiles successfully. Testing shows the same pre-existing
prerender error on /guides/troubleshooting as on master (supabaseUrl is
required).

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Upgraded MDX and markdown tooling to major releases (MDX v3,
next-mdx-remote v6, remark-gfm v4).
* Adjusted MDX serialization to disable embedded JS handling and remove
legacy dynamic-import behavior for more consistent rendering of docs,
guides, and code examples.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

apps/docs/package.json

@@ -43,8 +43,8 @@
   },
   "dependencies": {
     "@har-sdk/openapi-sampler": "^2.2.0",
-    "@mdx-js/loader": "^2.1.5",
-    "@mdx-js/react": "^2.3.0",
+    "@mdx-js/loader": "^3.0.0",
+    "@mdx-js/react": "^3.0.0",
     "@next/bundle-analyzer": "15.3.1",
     "@next/mdx": "15.3.1",
     "@octokit/auth-app": "^7.0.0",
@@ -91,7 +91,7 @@
     "micromark-extension-gfm": "^2.0.3",
     "micromark-extension-mdxjs": "^1.0.0",
     "next": "catalog:",
-    "next-mdx-remote": "^4.4.1",
+    "next-mdx-remote": "^6.0.0",
     "next-plugin-yaml": "^1.0.1",
     "next-themes": "^0.3.0",
     "nuqs": "^1.19.1",
@@ -108,7 +108,7 @@
     "rehype-slug": "^5.1.0",
     "remark": "^14.0.2",
     "remark-emoji": "^3.1.2",
-    "remark-gfm": "^3.0.1",
+    "remark-gfm": "^4.0.0",
     "remark-math": "^6.0.0",
     "server-only": "^0.0.1",
     "shared-data": "workspace:*",