Dependency: Update postcss-loader in Next.js framework

[valentinpalkovic]

Closes #26702

What I did

I have updated the postcss-loader dependency in @storybook/nextjs to support esm-based postcss configs. The postcss-loader major bump only removes support for Node.js 16. No other major breaking changes are documented.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

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core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

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[yannbf]

LGTM but here's a reference in case we need to check in the future for the breaking changes:

• PostCSS 8 dropped Node.js 6.x, 8.x, 11.x, and 13.x versions support. All these versions have no security updates anymore.
• We now serve ES6+ sources in the npm package without Babel compilation. If you are creating tools like CodePen and put PostCSS into the client-side JS bundle, you may need to run Babel on node_modules/postcss for old browsers.
• We removed rarely used postcss.vendor API.