chore: disable code block html preview (#12829)

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

## Summary by CodeRabbit

- **Chores**
- Updated the default setting for the code block HTML preview feature
flag; it is now disabled by default. This change affects only
experimental builds.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: L-Sun <zover.v@gmail.com>

packages/frontend/core/src/modules/feature-flag/constant.ts

@@ -280,7 +280,7 @@ export const AFFINE_FLAGS = {
     description:
       'com.affine.settings.workspace.experimental-features.enable-code-block-html-preview.description',
     configurable: isCanaryBuild,
-    defaultState: true,
+    defaultState: isCanaryBuild,
   },
   enable_adapter_panel: {
     category: 'affine',