Bump markdown-to-jsx and @storybook/addon-essentials in /app

Removes [markdown-to-jsx](https://github.com/quantizor/markdown-to-jsx). It's no longer used after updating ancestor dependency [@storybook/addon-essentials](https://github.com/storybookjs/storybook/tree/HEAD/code/addons/essentials). These dependencies need to be updated together.


Removes `markdown-to-jsx`

Updates `@storybook/addon-essentials` from 8.1.1 to 8.4.7
- [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/v8.4.7/code/addons/essentials)

---
updated-dependencies:
- dependency-name: markdown-to-jsx
  dependency-type: indirect
- dependency-name: "@storybook/addon-essentials"
  dependency-type: direct:development
...

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