Bump storybook from 8.6.4 to 8.6.15

Bumps [storybook](https://github.com/storybookjs/storybook/tree/HEAD/code/core) from 8.6.4 to 8.6.15.
- [Release notes](https://github.com/storybookjs/storybook/releases)
- [Changelog](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.md)
- [Commits](https://github.com/storybookjs/storybook/commits/v8.6.15/code/core)

---
updated-dependencies:
- dependency-name: storybook
  dependency-version: 8.6.15
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]