[8.14] Fix storybook config (elastic#182283) (elastic#182293)

# Backport

This will backport the following commits from `main` to `8.14`:
- [Fix storybook config
(elastic#182283)](elastic#182283)

<!--- Backport version: 9.4.3 -->

### Questions ?
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BACKPORT-->

Co-authored-by: Jon <[email protected]>

packages/kbn-storybook/src/webpack.config.ts

@@ -139,6 +139,9 @@ export default ({ config: storybookConfig }: { config: Configuration }) => {
     stats,
   };
 
+  // Override storybookConfig mainFields instead of merging with config
+  delete storybookConfig.resolve?.mainFields;
+
   const updatedModuleRules = [];
   // clone and modify the module.rules config provided by storybook so that the default babel plugins run after the typescript preset
   for (const originalRule of storybookConfig.module?.rules ?? []) {