chore: bump @storybook/addon-essentials from 6.4.19 to 6.4.21 in /site (#950)

Bumps [@storybook/addon-essentials](https://github.com/storybookjs/storybook/tree/HEAD/addons/essentials) from 6.4.19 to 6.4.21.
- [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/v6.4.21/addons/essentials)

---
updated-dependencies:
- dependency-name: "@storybook/addon-essentials"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

site/package.json

@@ -46,7 +46,7 @@
     "@pmmmwh/react-refresh-webpack-plugin": "0.5.5",
     "@react-theming/storybook-addon": "1.1.5",
     "@storybook/addon-actions": "6.4.20",
-    "@storybook/addon-essentials": "6.4.19",
+    "@storybook/addon-essentials": "6.4.21",
     "@storybook/addon-links": "6.4.20",
     "@storybook/react": "6.4.20",
     "@testing-library/react": "12.1.4",