Bump css-what from 2.1.0 to 2.1.3 in /fixtures/attribute-behavior (#25429)

Bumps [css-what](https://github.com/fb55/css-what) from 2.1.0 to 2.1.3.
- [Release notes](https://github.com/fb55/css-what/releases)
- [Commits](https://github.com/fb55/css-what/compare/v2.1.0...v2.1.3)

---
updated-dependencies:
- dependency-name: css-what
  dependency-type: indirect
...

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