Bump prismjs and @storybook/addon-storysource in /docs (#2829)

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


Removes `prismjs`

Updates `@storybook/addon-storysource` from 7.0.27 to 7.6.20
- [Release notes](https://github.com/storybookjs/storybook/releases)
- [Changelog](https://github.com/storybookjs/storybook/blob/7.6.20/CHANGELOG.md)
- [Commits](https://github.com/storybookjs/storybook/commits/7.6.20/code/addons/storysource)

---
updated-dependencies:
- dependency-name: prismjs
  dependency-version: 
  dependency-type: indirect
- dependency-name: "@storybook/addon-storysource"
  dependency-version: 7.6.20
  dependency-type: direct:production
...

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Author: dependabot[bot]