Bump postcss and @storybook/web-components in /mail/components/storybook

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [@storybook/web-components](https://github.com/storybookjs/storybook/tree/HEAD/code/renderers/web-components). These dependencies need to be updated together.


Updates `postcss` from 8.4.17 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.4.17...8.4.31)

Updates `@storybook/web-components` from 6.5.12 to 7.4.6
- [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/v7.4.6/code/renderers/web-components)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@storybook/web-components"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

mail/components/storybook/package.json

@@ -23,7 +23,7 @@
     "@storybook/addon-links": "^6.5.12",
     "@storybook/builder-webpack5": "^6.5.12",
     "@storybook/manager-webpack5": "^6.5.12",
-    "@storybook/web-components": "^6.5.12",
+    "@storybook/web-components": "^7.4.6",
     "babel-loader": "^8.2.5",
     "lit": "^2.3.1"
   }